When you try to sign in to Metatask, it always asks for your team's domain first. If you forgot your team's name, you need to click on link button "Find your team" right under the sign in form.
Then you need to enter the email address you use for Metatask, and press "Send email". You can enter one or several email addresses if you are not sure which one you used for Metatask registration. If you used any of them for Metatask sign-up, your team name will be sent to the address(es). You won't get any email if you didn't use these addresses for Metatask registration.
This email will contain your team name (or multiple teams) and URL address. You can sign in to Metatask clicking the green button on this email (it contains your team name), or using the standard sign in form.