Can forms plugin process differently depending on a condition?

I have a simple contact form. I want to add a checkbox to enable someone to optionally subscribe to an email list when they submit the contact form.

Is there a way I can look for whether the checkbox has been checked, and have an additional form action if it has been?

Thanks in advance!

Thanks for your reply! I found grav-plugin-mailchimp, which is not in the GPM, but works well. I don't need any extra logic, either, if I just use required_fields:, and reference the checkbox, in the mailchimp form-processing portion. That way, if the checkbox is checked, the mailchimp plugin action is called, and if not, it isn't.