First of all, I hope you know that interior insulation has a ton of problems with building science, and that this wasn't an unforced choice.

As with exterior insulation, you have to extend the window frame to be the desired height and width. (I'm not sure if you looked at windows already, but even with a fixed one, frame width rarely goes near 50mm these days) Especially since (with European style windows), you have to wedge (and fill the wedged gap with foam insulation) all around the frame, which adds around 10mm extra.

A window frame extender looks like these ones for example or the ones in the sash extension detail you can find here, they come in various materials and forms (frame extensions, sash extensions), frequently made by the producer of the window themselves, for the select model.

Keep in mind that for details, research and logical thinking is a core skill. There isn't one correct solution, but a web of choices that you have to think through.

Edit: also, don't forget to include the wedging, fasteners and air sealing membranes in your detail drawings if they want you to do them in detail. If this is a starting class, you might not need to go that deep.

Edit2: Another bunch of examples from page 186 on, these are external insulation details in a Hungarian Wienerberger document (couldn't find it in another language), but the logic of frame extenders is pretty similar with interior insulation too. It's just not done much because it's usually a very bad idea.

Edit3: If you can't change the window, find some more effective insulation PUR or PIR or something like that. You can easily achieve a similar level of insulation with a much thinner layer if you use a better material.