Hi @martijnmpmnf5, welcome to Thunkable! 
Be sure to check out our posts about How to ask Great Questions v2.0 and our Community Guidelines as you get started.
I am not familar with a Servo but from looking at this site, you can connect via BLE to the servo and simply send out the angle value that you want the arm to turn and it would.
Keep in mind, with Thunkable, you’d have to use Bluetooth Low Energy to connect to the board as we unfortunately do not currently support connecting via WiFi.