The form plugin for Grav adds the ability to create and use forms. This is currently used extensively by the admin and login plugins.
The form plugin is easy to install with GPM.
$ bin/gpm install form
Simply copy the user/plugins/form/form.yaml into user/config/plugins/form.yaml and make your modifications.
enabled: true
The Learn site has two pages describing how to use the Form Plugin:
Note: when using email functionality in your forms, make sure you have configured the Email plugin correctly. In particular, make sure you configured the "Email from" and "Email to" email addresses in the Email plugin with your email address