Help with dynamic nested JSON in API

I’m struggling with sending POSTed data in a nested JSON via server connect API.

My workflow is: Server Connect Form >> Database Insert >> API Action

I need to replace “John” with the value from the form input.

         {
              "From": "sender@example.com",
              "To": "receiver@example.com",
              "TemplateId": 1234,
              "TemplateModel": {
                  "user_name": "John"
              }
          }

So far I’ve tried to use the data binding in the JSON window to set it as this: “$_POST.firstName” but when I click save, it changes to this: “[[{“id”:”$_POST.firstName",“title”:"$_POST.firstName",“value”:"$_POST.firstName"}]]"

Any ideas?

Screenshot 2020-04-11 at 00.33.231052×1658 189 KB