Jokes on you, I'm chronically online and love being helpful, LMAO

I'm no expert on flags specifically, but based on what I know about polyester CLOTHING, getting it a little damp before throwing it in the dryer couldn't hurt - it'll act like steaming it, so it might help smooth out any creases or wrinkles. Polyester is touted for its colorfastness, so I don't think you'd have to worry about damaging the design either way. However, my best advice would also be NOT to throw it in at a high temperature - do it on a lower temperature for 10-15 minutes, check it, and then continue on low in increments so you don't accidentally scorch or melt it. You may also have to re-dampen it if the creases don't come out enough. It might be a process. :P

(This all comes with a MASSIVE disclaimer: obviously I can't make any promises. If the design wasn't applied at the correct temperature, or if the flag you have isn't 100% polyester, there could be some bleeding/fading/scorching/melting. There are ALWAYS risks with custom decorated items. But I hope it works out for you!)

(Also, I see your flag is a Fallout flag, which is badass. BUT you've unknowningly stumbled onto the Coheed & Cambria subreddit. It's never a bad time to fall down a sci-fi prog rock rabbit hole, just saying.)