Not really clear, what you are trying to achieve, when you say „top left of the screen":
Guessing, what you intend to do, I’d suggest to try something like this:
Place a text frame (A) with the background colour you want.
Place a second text frame (B) and stretch this one to browser width.
Copy the first text frame (A), and place a text cursor into the second one (text frame (B)).
Paste the text frame (A) just like text into text frame (B).
Make sure, that the text in text frame (B) is set to left aligned and use the „Text“ panel to define a left magin to make sure, that the content isn’t placed directly at the left edge of the browser window.
Got it now! Thank you!