First, Tuba asked me to enter the name of the server I use. So I typed in "mastodon.nzoss.nz". It was happy with that (no "https://" required) and redirected me to the default Masto app in my browser.
I was already logged in, so I landed on a page asking me to approve Tuba for reading and posting to my account. I clicked approve and (after giving my browser permission to open links in Tuba) it returned me to the app. I got a confirmation message that it was set up and ready to use.