Ho set a variable in a object Json

Hello everyone, since then I am trying to send a variable to a json object but its pass. I explain. I have an image which I convert to base64 which I want to assign to a variable which will be sent on a json object. on the header I have an api_key that I set its value. when with postman or ARC its progress it returns me as status code 200 but on uipath its progress. please could someone help me?