If successful returns the same enrolment information as GET/enrolment/​{LoginID}/​{EnrollID}
If course exit doesn't return the user to your application then append the LaunchURL with &returnURL= plus the URL of the page to be loaded after the course exits.
If POST/enrol/​{LoginID}/​{CourseID} returns an error containing multiple enrolment options found then use POST/enrol/​{LoginID}/​{CourseID}/​{DurationID} to also specify a DurationID from those listed in the AvailableEnrolOptions in the response from GET/catalogue/course/​{CourseID}/​{LoginID}