I accidentally entered my password in the username field of the captive portal and that now comes up as an auto-fill suggestion for the username, which I would like to delete/reset. This does not open up in the browser, but does so in some separate default window which I don't seem to have much control over. Is this possible?
Go to Settings > Apps, and show all system apps.
Find the CaptivePortalLogin app in the list and clear all the data for it by going to Storage > Clear Data.