Most REST API calls require a payload using the HTTP method POST to post the payload to the server through a REST API call.  Here's a common REST API method that creates and executes a Process instance based on a Process definition.

The above REST API documentation for the method “Execute” shows the following:

• Http method - POST
• URL - /api/Process/Execute
• Request body or payload in JSON format

{
  "isTestMode": true,
  "testModeUser": "string",
  "definitionID": "string",
  "instanceID": "string",
  "instanceName": "string",
  "variables": {
    "additionalProp1": "string",
    "additionalProp2": "string",
    "additionalProp3": "string"
  },
  "globals": {
    "additionalProp1": "string",
    "additionalProp2": "string",
    "additionalProp3": "string"
  }
}

The above JSON payload describes the payload structure, keys, and values, including the value data types. Wherever it says “string,” we suggest passing a value to it; otherwise, pass an empty string, do not pass the value “string.”

Here's a valid payload to send, all you have to provide is the Process definition ID.

  "isTestMode": false,
  "testModeUser": "",
  "definitionID": "3536a1fe-8825-4929-8c59-b3a73f5e7ffc",
  "instanceID": "",
  "instanceName": "",
  "variables": {
  },
  "globals": {
  }
}

Here's the POSTMAN configuration for the REST API Call:

The above request was sent to the server, and the server processed it successfully and returned a JSON response.  This call is under security, so provide your username and password for authentication.