How do I sign in on Microsoft to do with another account?

I bought a new laptop including Microsoft Office basic 2021. I installed Microsoft to-do and signed in with the account that is connected to the Office basic 2021 account. I would also like to use Microsoft to-do with an account that does not have a license. The account however does work with the app Microsoft to-do on my iPhone therefore I think it should work on my laptop as well.

Another thing, even if I try to add another account in the to-do app it does not give me an option to sign in. If I sign out from the account I can log in again but have to delete the app and reinstall it.

Can someone help me with both problems?