I would like to have an "autofill" so that every time windows asks for my microsoft account information it already has it.
Is there a way to do this?
This occurs if you've setup an account as a local account. I belive switching from local account to the Microsoft account results in the credentials being securely stored, allowing Metro apps to access them and hence you can install from store without having to re enter the password.
You should be able to switch from Settings → Users and clicking on "Switch to Microsoft Account" as exlained here