Type 'easy' or 'hard' to begin the quiz. Easy is a 15-degree grid, hard is a 10-degree grid. Then, try to name the largest 50K+ cities in each square IN ORDER. To name a city, you must first click on the grid square to make it have a GREEN border. Then, type in the city answer for that square followed by a '.' (a period).
The squares will start with '0/5' (or maybe fewer cities if less exist in that square). Click on a square and enter the largest city in that square not yet answered (so, the largest city if you have yet to answer a city in that square). You will then have '1/5' and need to click on that square to name the 2nd largest city, and so on until you complete all the cities up to 5 (or less if less exist). If you name an incorrect city in a square it becomes 'locked' and turns red and you can no longer accumulate cities in that square.
You receive 1 point for the largest city in a square, 2 for the 2nd largest, and so on.
At the end of the quiz the largest remaining city you haven't named will be shown in each square.
Quiz Takers 
Is there any reason I’m not aware of why Tromsø isn’t included? Current population looks to be over 50k but that might be a similar situation to Muscat I don’t know of.
You might be looking at the full municipality – city proper is a little too low:
https://citypopulation.de/en/norway/cities/ua/?cityid=8252
Saigon (Ho Chi Minh) should be accepted under both names as they’re equally used and thus should be on both the 6 and 9 letter quizzes.
Also, since Mumbai/Bombay is a 6 letter city no matter which of those two names you use, both should be accepted.
On most quizzes Saigon is accepted as an alternative for Ho Chi Minh City but for this letter-based series it must be one or the other as no city should appear on more than one quiz. As the official name for the city is Ho Chi Minh City that name should prevail over Saigon which, as useful as it is as an alternative name, it’s an outdated designation.