API raw json status 400 code INVALID_TYPE

I keep getting a status code 400 INVALID_TYPE VALIDATION_ERROR using the Bubble API Conecctor plugin. I tried the file in Postman which is working correctly.

JSON file i would like to post (all variables are of type string according to the documentation)

This is the response code:
Raw response for the API
Status code 400
{“code”:“VALIDATION_ERROR”,“message”:“One or more validation errors occurred.”,“validations”:[{“message”:“Required”,“code”:“INVALID_TYPE”,“property”:“start”},{“message”:“Required”,“code”:“INVALID_TYPE”,“property”:“end”},{“message”:“Required”,“code”:“INVALID_TYPE”,“property”:“bookingContact”},{“message”:“Required”,“code”:“INVALID_TYPE”,“property”:“rooms”}]}

Any suggestions?

Add header for Content-type with value to application/json