Dropdown Field populated with database values

I'm looking for some help getting a dropdown field populated with values from a database based on the selection of a value selected in a radio field. I have my PHP code and Javascript created, and eveything works. But, I'm stuck on getting the dropdown field to register the selected option from the listed values (from the database). Looking at form processes, I'm seeing the admin-ajax.php file locked (423)? Wondering if there is a thread covering this or if there is a help document?