The simple process of setting item level permission is not available for SharePoint 2013 workflows. The only way I was able to do was through REST api called under Appstep.
There are 2 calls made:
This blog by Yuri Leontyev over at spssite.pro can be a good start.
BreakRoleInheritance is just not documented and can be used using POST request:
where copyRoleAssignments – “if true, this method copies the role assignments of the parent securable object when breaking inheritance; otherwise, this method adds the current user to the permission level that is required to manage the list item.” clearSubscopes – “Indicates whether subscopes should be cleared or not.”
Instead of the Add there is AddRoleAssignment method that also can be called using POST request (NOTE: this method works incorrectly in RTM release of SharePoint 2013!!!! It checks user within current item and if user is not found (usually as we’ve just broken inheritance) throws an exception. In SharePoint Online and March CU it works as expected):
where principalid is id of user or group within current site collection. You can get a list of users by executing using GET:
roleDefId is internal id of Role Definition (Read, Contribute, Full Control and so on). You can get a list of available role definitions using GET:
Please use this guide to Enable App step in SharePoint 2013 Workflow, once you make sure all is set up well, the above solution will work fine.
Answer copied from How to Set Item Level Permission Using SharePoint Designer 2013 Work Flow?.