I have a requirement to start a workflow from outside SharePoint. If any REST API exists to start SharePoint designer workflow.
Can anyone help me with this, which API should I use for this?
Hope this meets your requirement you trying to achieve with Rest API
If are you looking for how to trigger workflow to start from external to sharepoint environment via REST API, I have shared how you could do taht in sharepoint online on office 365 by calling the REST API at _vti_bin/client.svc/ProcessQuery, basically you will need to do HTTP Post at the end point twice, one to get the workflow subscription and the other to trigger the workflow instance to start, find it here https://community.nintex.com/community/tech-blog/blog/2017/01/18/stronger-together-nintex-workflow-cloud-office-365
The same REST end point could be used in sharepoint 2013 as well.