Flask server create ondemand change event for dropdowns

I have example HTML and Flask app where in HTML there are 2 dropdowns (Dropdown 1 and Dropdown 2) that GET their values from Flask server. Now, I wanted to add JS onchange event to pass value selected by user from DROPDOWN1 to the server without submitting. The resulting post to server of Dropdown1 (from on change event) is used to filter only applicable values of DROPDOWN2 list in the HTML. I want this to be integrated in my code and explained. This is a small ask for anyone who knows JS. I have max $10 budget for this.