Hello subtenous, my name is Emily, I haven't earned any titles here, but I can try to offer my aid regardless.
Firstly, are you signed into Windows with a Microsoft Account, if you are, this in itself should sign you into the Microsoft Store, and you should be able to close that window safely, and be signed in anyway.
If you're not signed into Windows with a Microsoft Account, this may be worth doing. To do this, go to Settings, Accounts, and click 'Sign in with a Microsoft Account instead', doing this should sign you in to all Microsoft applications, including the Store.
If none of the above methods work, reply to this message, and I'll try to help with other methods.
Feel free to contact me with any queries
-Emily Lowe