Unable to configure the header and json data

Hello all,

I need to connect to a web app that supports REST API that is currently not on the list of Workato. 

The authentication is simple: 

 - I need to make a POST request to API url

 - I need to attach the API key to the header of the post: API_key

 - I need to attach my login information which contains 4 fields: UserName, UserId, Password, OrganizationID

If login successfully, the app will return a json containing a Token key. 

I've been struggling with where to configure these parameters so that they'll result in proper request to the API. I tried a lot of different things but I just get an error message: "{"error":"Webhook configuration doesn't match request method"}". Also, when I click Test my Recipe, it doesn't say whether the testing is successful or not. 

Can you anyone help me? 

I suspect my problem might how I put in the Json info. What is the coding convention for the Json? For example, on my Python, my login information is coded as:

data = {

        'UserName': "XXXXXXX",

        'UserId': "XXXXX",

        'Password': "XXXXXXXXX",

        'OrganizationID': "XXXXXX",

    }

What should it be on the Request Body?