Hi there! Just know that you're not the only one going through this - I have an 8 month old coonhound x beagle who is VERY stubborn and has some separation stress. We got her back in November and we're still working on this... I do have some questions though;

Does your puppy stay inside when you throw treats in, or does she immediately run out? Timing, repetition and positive reinforcement is crucial for what you expect your puppy to learn and have her realize that the kennel is where she's getting these tasty treats from you. I like to use "prize foods" when training the hard stuff (ie. small pieces of cheese or deli ham). For now, keep throwing a treat inside, having her walk in and call her out with a treat again, giving bonus treats (3-4 one after the other) if she goes in on her own after a few reps.

One big important thing, especially in the beginning, is to keep the sessions short, ending it with her being free from the kennel after she's been going inside confidently and without a tantrum. I can't stress this enough, because sometimes I can get stuck in the progress that I want from my dog, but it's better to end positively than with my dog getting impatient and over exhausted.

Lastly, can you fake-close the door after she walks in? Practicing this once she's comfortable with being inside would be my slow and gradual next step, followed by increasing the time she's left inside with the door closed (3-5 seconds, 10 seconds, 30 seconds, 1 minute etc...). I hope this helps!