This is a focused task and the key to entering access to the content.
Some choose to show some content to pull the user in, but lowering the cognitive load is important to a focused task.
Some choose to hide all content because it should not be viewed without privileges.
If you go with the first approach, deciding how far to lower ithe cognitive load ...
In order to minimize the number of stimuli - at the moment when less information reaches us, we are able to focus on the goal.
Exactly the same with, for example, shopping processes to reduce the page drop rate during the process.
It's correlate with reducing memory load:
App has a clear structure
“Recognition over recall”
What problem are you trying to solve?
While the form you are proposing is certainly technically possible, it is not clear how it improves the overall user experience of logging in.
Login forms do have many faults, but the user's ability to click the button is generally not one of them.
While this design could possibly save someone the cognitive burden ...