I think there must be dozens of posts like this on this forum.
And they all get the same answer.
Use an email account to create an account. Preferably not a gmail account because Google absolutely hates free services that it cannot get a cut from, but even gmail works. Use that email and set a password, click register / login. That works both in the desktop and mobile app.
However, you CANNOT create an account from the website - there's no point to that... the account is for Subsurface cloud storage, not for a web thingy, so you create it from one of our apps.
You'll get an email with a PIN code within minutes. Your email provider will do a lot of work to hide that email from you. Especially Google, see above.
Then you enter the PIN and forget about it in the future, because now your account is enabled.