Hi TC, sorry I think I missed a chat from you last night as well. I've just added a new content type option on the custom webhook call step. I've done a quick test and it runs successfully, but I don't have a target webhook to actually test it on.
Can you try this for your device and let me know how it goes?
You should specify the parameters in the body field, using something like:
device_id=1234&brightness=50