Hey! I recommend you to use API Form action, you can put there all the inputs you need, if json is selected, then the payload will be that way 
Also you can use something like beeceptor and check what's sending:
As you can see, a valid json
If you need to send some cookies on headers, just add credentials="true" on the code, please see: https://community.wappler.io/t/credentials-flag-on-app-connect-api-form/62205/1