Coding the droplist function

Hi there,

I believe this plugin is very well developed and professional among other plugins,

however, i cannot find a documentation that clearly shows or teach how to code the drop down list for the configuration form. Can you please advise? thanks alot.

i am at Manage Database Fields -> form element -> dropdown list and i just cant happen to find the correct coding to make the form. Thanks

https://wordpress.org/plugins/participants-database/