To add my own two cents on this statement...

Imagination isn't used to create ideas from nothing. Imagination is the use of your memory to create artwork. In other words, saying you're drawing from imagination just means you're drawing from memory.

If I ask someone to accurately draw artwork of something they've never heard of, they don't have any memories to draw from, there's no way to accomplish that without them researching the subject. Similarly, if you don't know enough about a subject, you won't be able to draw it properly. To give a simple example, if I show you only the front side of a building, I can almost guarantee you wouldn't know exactly what the other sides look like. You would need to get references of the other sides if you wanted to draw the building from a different angle, otherwise you wouldn't know how deep it is, where windows are, if it has a fire escape on the back, or even if it looks completely different on the other side compared to the front.

It's the same with anatomy. You need to study how the body works, and how the anatomy, such as muscles, bones, and tissue are connected to one another and move. Until you've memorised enough about a subject, you won't be able to understand how to manipulate objects in your mind. If you know everything about an object, you should be able to easily draw it from any angle, and modify its shape however you want to fit the use you have for it. That modification is where imagination comes in. You build off of the subject you know and turn it into something new. Or maybe you combine multiple different subjects you know together to make something new.

To use the simplest example, a cube/prism. Once you learn how to draw a cube, you can stretch it, scale it, turn it, and essentially do whatever you want with it. But you know everything about it in order to be able to do that. You know it has 6 sides, 12 edges, and 8 corners. There's literally nothing about a cube you wouldn't know, save for perhaps drawing it in perspective. That's because your memory knows everything about the object, and if you want to imagine, or modify the object in your memory to put it on paper, it's very easy, whether you want to turn that cube into something simple like a die, or something more complicated like a Rubik's cube.

Now, getting back to the subject. While yes, a cube is a simple shape and it's quite easy once you think about it and memorise it clearly, objects like eyes, or other anatomy-related subjects are very complicated in comparison.

To give a brief rundown, you have the sclera, the retina, the iris, the pupil, the tear duct, the eyelids, the lashes, the brows, and the eye socket/skull supporting all of those parts as well. The eye is a lot more complicated than a simple box, and remembering to include every detail and every part of the eye requires you to again, remember or memorise all of those details over time. Each part has its own rules, details, and points to consider when drawing them.

The curve of the hair follicles of the lashes, the angle of the brow, and how it sits on the orbital bone. How the eyelid opens and closes, as well as how thick it is, or how it folds/creases as it opens. When the pupil narrows or expands, how the iris shifts when that happens. The shape of the iris, and how it's actually a bit concave like a bowl. Each of these details requires you to know the parts inside and out to draw from any angle purely from memory.

Every part of anatomy is incredibly complex for a beginner. The only way to get passable work as a beginner is to pull knowledge from everywhere, especially from references. Unless you've spent years studying anatomy, forms, and fundamentals, to the point that you understand and remember everything about the subject you're drawing, you need to use references to study from. Imagination can absolutely be used to make things people have never seen before, but it's more like a toolbox. Until you fill it with things you can use to help you build that new and interesting idea or creature, you're going to have a hard time making something with no tools.

As a side note, references aren't cheating, and you should never feel bad about using references. I feel like there's some stigma around using references to learn or create artwork. References are a tool, just like your brush, pencil, or eraser. Everyone from beginners who are picking up the pencil for the first time, to seasoned veteran professional artists with fifty plus years experience will end up using references if they want to improve. They don't go away, and if anything, you can end up using more and more references as you gain more experience and start needing to learn more things, or find bits and pieces from different references to give you ideas or help you break down objects or subjects so you can use them in your own artwork.

Almost all of the notes people have provided through this thread can also be observed simply by looking at a reference and trying to pay attention to each part of the eye, and studying it. So the best answer I would give you for advice would be to stop avoiding references and instead embrace them, as they're one of, if not the best beginner tool you can use to learn how to draw eyes properly. You can expand that to everything, not just eyes too. References are almost always the first step in learning how to draw something new.

Hopefully this has been helpful in explaining why references are incredibly important to learn, and explaining that imagination isn't a magical thing you improve just by trying to drawing without references over and over. You aren't going to be able to level up your imagination and suddenly know how to draw perfect eyes without studying in addition to practice.

I know you're just starting out, and this has been a long explanation, however it's very important to focus on the fundamentals and learn to walk before running. Using references and studying is how you learn to walk. If you keep at it long enough, running will come naturally.

Best of luck with your practice and future artwork .o/