I've recently been concerned about this. If you're logged in to YouTube on a SkyQ box, this logs as a Linux machine with an image of a Laptop in your Google Account. I just kicked it out, logged it back in to YouTube and the same Linux machine reappeared.
If you don't know the administrator email address you have to do the regular account recovery procedure for Google accounts but you might have an additional option that is to demonstrate the domain ownership. For details see Reset your administrator password
How do I recover my Google account (or Gmail) password or username?
Why doesn't G Suite ...
Copy the link of form and then go in incognito mode.
Paste the link on the search bar and then once it will open it will ask you to sign in again.
Now click on sign in. It will ask you for Gmail I'd. Fill the I'd then fill your password.
This works, and your form opens without any worries.