I am trying to call a nintex workflow using Call HTTP web service action instead of Start Workflow action.
Please see Component Workflows from the help documentation.
Use the Component workflow option to start your workflow from another workflow or HTTPS-capable service or application.
You can start a component workflow using:
Any HTTPS-capable application, by sending an HTTPS Post request
The workflow or external application that starts the Component workflow must provide the start variables defined by the Component workflow.