You can easily let people register themselves for the volunteer pool / volunteer bank using the following piece of text: /register?next=%2F%3FvolunteerOnboard%3Dtrue%26volunteerOnboardRegister%3Dtrue. If you place this text behind your domain name, you have a complete URL.

The final URL could then look something like this:

Depending on where you place it (as a link or as a link in a button) you need the full url or just the last part.