There are two possible ways
If the Workflow Role is a EBS responsibility, you can simply assign the responsibility to the FND User. (i.e. this can be achieved using the application forms).
If the Workflow Roles is not a EBS responsibility, i.e. it has been created as an ad-hoc role using the WF_DIRECTORY.CreateAdHocRole API then you need to assign the role to user also using the WF_DIRECTORY.AddUsersToAdHocRole API. (There is no way to achieve this using application forms, it can be done only using API).
