Open Google or Youtube in Edge an you'll see a huge banner advertising Chrome. That use to happen for practically every other browser.
That's the premise behind antitrust (even though in practice, its 50 years behind the times). If you have dominant market share, things that are otherwise lawful^ aren't anymore.
^Antitrust stuff like anti-competitive behaviour isn't criminal regardless.
It would be anti-competitive in your hypothetical if your bank refused to make transactions to your auto insurance company because it was not the bank's insurance company.
Pop-ups in your bank portal are uncomfortable but are not anti-competitive.
Popularity != Monopoly