How do I report players in Halo: The Master Chief Collection on PC?

On Windows 10, you can report recent players through the Xbox Console Companion app.¹

To report a player that behaved in an inappropriate way or has an inappropriate name or Gamertag:

1. Navigate to the Friends menu. If the player is not a friend, select Followers or Recent players from the drop-down menu to bring up the relevant list. You can also search for the player by typing their Gamertag into the search bar.
2. Select the Gamertag you wish to report.
3. Select More, and then select Report.
4. Select a reason for the report.
5. Provide any additional information in the text box, and then select Report to Xbox.

Source: Xbox Enforcement - Submit a report on the Xbox app

^{1Previously known as the Xbox app before being rebranded in June 2019.}