You should be able to do this right along with the other game options: standard, days per turn, random and rated. Above where you choose this, you will see a box with tabs along the left side. If you choose a Random opponent, the center box will populate with settings you can configure, like this: 
The default goes to 'Member' and if you don't enter a specific member's name, one will be picked at random for you. Let me know if this doesn't help or if you have further questions. 
To start a new online chess game I go to Start new game which gives me some options of what kind of game to play; standard, days per turn, random and rated, (and starting position under ‘more settings’)
If I start a game and am not matched immediately, I get the option to edit my seek, this gives me opponent settings; choose their rating, number of games played, move speed etc.
Usually of course, I’m matched up immediately based on the original seek which means I don’t have time to edit these settings. Does anyone know how I get these opponent settings in my original request?