1. Make your game 2. Create execuable 3. Save the file 4. Use This download 5. Drag saved exe. to blue area 6. When done with download, go to new game file, click properties, and check if its an swf. 7. Post your game!
There even was a project started by a few people with knowledge in GM and a few people with knowledge in AS2 called G-Script, which was meant to convert .gm8 (game maker file) to .fla (flash).
1. Create game in GM 2. Make sure game is really good 3. Contact a flash developer 4. Ask said developer to port your game 5. Hopefully the developer does 6. Done.