Stijn007,
I can't see what you have in your Compose steps but it looks like you still have to call an actual Message ID. These triggerBody()?['id'] evocations need to link to some variable in the logic of your Flow, it doesn't look like you're doing that yet.
You may have noticed in the linked post how the three Compose steps were split up because the Message ID was intentionally excluded. This is crucial because the Message ID is a dynamic variable, whereas the rest of the link is static.
The expression should probably be more like this...
concat(outputs('Opstellen'),triggerBody()?['Message id'],outputs('Opstellen_2'),triggerBody()?['Message id'],outputs('opstellen')triggerBody()?['Message id'])
Note that I'm using 'Message id' because that should be the name of the dynamic variable you instantiate from the Get Messages step earlier in the Flow. You should be able to just plug that into your Concat() function and get this working. You could otherwise use Initialize Variable to import the Message ID, sometimes that works better.
I hope this helps!
Patrick