Dear Laurin,
Unfortunately, the ability to actuate the TMD through Simulink is not yet a standard feature of FAST v8 or OpenFAST. But I’m sure this feature would be possible to add through customization of the source code.
I had a dialogue with Xing.Wei on the forum regarding this topic–see: Question about running FAST 8 in Simulink. You may want to reach out to Xing.Wei to see if he was successful and can share his updated source code.
Best regards,