You can trigger WR API from your Salesforce Flow.

Create a SalesForce flow, add all necessary steps. 

Create new action > Type in 'API' to search input and select Work-Relay API

Provide action Label and API Name > check 'Include' for 'Set Input Values for the Selected Action' setting > provide parameters. 

Please follow below template to pass parameters to the WR_API class:
"class" - API class name
"method" - API method name
"parameters" - parameters in a JSON format

Example params string: {"class":"WorkflowService", "method":"changeAssignee","parameters":{"cursorId":"{!cursors_loop.Id}", "assigneeType":"User", "assignee":"{!$Record.Assigned_User__c}"}}