Well, using what I explained before, just add a checkbox to save the info typed in the login screen. If the checkbox is checked, save the login info (username and password entered) in a TinyDB.
Next time, when the app starts, check if there's any user info saved in the TinyDB. If there is, populate the username textbox and the password textbox with what you have saved.
Then let the user press the login button or change the login info at will.
But I'm still not sure if I understand your question.