How is API request body created?

I have this put API.

URL looks like this : /v1/xp/generalUsers/{generalUserId}/users/{userId}/apps

but Request body looks like this :

  {"connectedApps": [    {"connectedAppId": "string","applicationRoles": [        {"roleId": "string","assignedResources": [            {"resourceType": "string","resourceId": "string","resourceHint": "string","resources": [                null              ]            }          ]        }      ]    }  ]}

I have another API call which demands the same exact parameters. But, I do not know its Request body.

I'm interested how and where is this JSON structure built.In code(.Net), I can only find the parameters that are passed to API call. I'm building URI and passing it.

I don't have Swagger for it.

To summarize,

• How can I find the request body of the API call and how is it built?