Set default location for document filename

When I click select for a document filename field, I want it to look and open a specified directory for that file. Further, I want to be able to dynamically set the directory where it is looking.

For example, I have radio buttons 2020 and 2021. If I click 2021, it would change my retrieval directory to c:\raw\2021. Of course if I choose 2020 radio button and click select on my doc filename field it should look in c:\raw\2020. My default document path for this form is c:\raw. Where/how do I accomplish this?