Current REST API Support
Get linked tasks: https://help.tableau.com/current/api/rest_api/en-us/REST/rest_api_ref_flow.htm#get_flow_run_tasks1
Run linked task: https://help.tableau.com/current/api/rest_api/en-us/REST/rest_api_ref_flow.htm#run_flow_now1

Add REST API Support
I don't believe the ability to create and configure a linked task is currently supported by the REST API. A linked task can be configured when on the page (https://your_server/#/flows/2490/scheduledTasks) displaying a specific flow by clicking "Scheduled Tasks" and then "New Task." Additionally, server.tasks.get and similar strategies for retrieving task info on the server using TSC currently defaults to only retrieves extract refresh tasks (task_type='extractRefresh').