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.
Add Chrome Cleaner extension to your chrome, here is link: https://chrome.google.com/webstore/detail/chrome-cleaner/cfghobccbjecjknfimipkglihchndkee?hl=en
Open the 'Chrome Cleaner', go to 'Cleaner Options'. On the top you will see 'Remove all browsing data from', select the required period. Check the box `Form Data' and clean.