They currently have a J fare (as opposed to full fare C) on the BKK-HKT route.

Domestic TG flights cannot pre-assign, and I've never used the online check-in. In order to get a F seat, you'll have to be one of the 1st checked in on the flight. GO early, and ask for an F seat. If it's avialable you'll be given one. But don't be surprised if they're already taken.

Star Gold doesn't make a difference in these seat allocations. It's 1st come, 1st served.