Create form with database connection

Hi,

just downloaded the theme and i’m having an hard time figuring out how to make this:
I need a form with a select and a result displayed after the button pressed.
I’m trying to add the select since i need this select to be populated via DB connection using PHP.

Is there a way to add the select using this theme? And after that, how can i add the PHP i need to connect to the DB and make it work?
I tried to modify function.php and also using PHP snippets but also a simple code as a function that displays “Hello World!” doesn’t work.