Add Tags Field to Event Submit Form

yes, you need to enable event tags at Events > Settings > General > General Options > Enable tags =Yes

currently the public submission form doesn’t accept tags yet, we do intend to implement it in the future though!

Is there a way to input a custom attribute and then have that automatically become a tag?

You could do that with some custom code. You could use a function that on a user submission (or approval) creates a tag using the value of the custom attribute and then deletes the custom attribute (assuming you don’t want to keep it).

Does anyone have any resources that would help me figure this out? I know some PHP and am familiar with WordPress but have never done anything like this before.

maybe you can try to modify event form at events-manager/templates/forms/event-editor.php and then add tag metabox

to use templates: https://wp-events-plugin.com/documentation/using-template-files/

other useful links:
https://shibashake.com/wordpress-theme/add-tags-and-categories-to-your-wordpress-page
https://wp.tutsplus.com/tutorials/plugins/how-to-create-custom-wordpress-writemeta-boxes/
https://wordpress.stackexchange.com/questions/26036/tags-as-a-dropdown-with-set-tags