Hey @apple,
A couple of things that might help...
User karh created a custom response extension called 'Better Response v2' that looks like it will do what you need. You'll just have to set the custom header content type that you want. Check it out here: 🎉 BetterResponse v2
While it would be a nice feature to have natively, I can imagine it will create a support nightmare for the Wappler team if users can play with the response options. Maybe the extension is the better way to go.
On a side note, generally you shouldn't have to include "OK" as plain text for success - the generally accepted standard is to send the correct http status code, 200=success, and the other end of your webhook looks for this; or whatever other code that best describes the error/issue.
Hope that helps