Custom File Requests? Javascript injection

Hello there,

I wonder if there is an option to create a custom file request form? Or some way to insert javascript code into the form itself?

Explanation:

Let's say I have a File Request form with 2 fields. Both fields are dropdown type. I need to dynamically populate dropdown number 2 based on the selected value from dropdown number 1.
This dynamic populating should happen by calling a specific API created on my end.

Is that possible? I'm not sure if there is a webhook for that, but I read somewhere that it is possible to insert a javascript code that will solve that problem with AJAX.