The first questionis whether LR has a native profile for the Samyang; I am guessing not, for reasons I won't spell out here. In any event, you should download and install the Adobe Lens Profile Downloader and see if anyone has created and uploaded a profile for the lens. Note that a profile created on any brand of camera with the same sized sensor should give you correct distortion correction, but perhaps not correct vignetting and CA because of the difference in sensor microlenses/cover glass/AA filer.
If there is none, you can create one for yourself using the Lens Profile Creator, it's actually pretty easy to do.