Sorry, we can only provide general developer guidance, unfortunately we can’t help with reviewing custom code in detail and implementing custom features as this is outside of our support scope. We plan to add the AJAX search feature to drop-down fields, but there’s no such option yet. The officially recommended solution for this issue is using Geolocation, then you can rely on the map provider API instead of creating a custom plain list of locations.
Please note that for the “source” parameter to work, the field type should be “select”, and this parameter should point to a URL that returns a list in JSON format similarly to how this function returns it (it’s a sample REST API endpoint for selecting listings in the drop-down) hivepress/includes/controllers/class-listing.php at master · hivepress/hivepress · GitHub
Hope this helps