How to prevent fridge freezer drain from freezing over?
There should be a rubber drain line that goes from the bottom of the condensate tray in the back of the freezer (probably behind an access panel), out the bottom of the refig, to the drain pan that's under the refrig.  That drain line can get clogged with dust/dirt or spider webs.  When this happens, the water can't flow out of the freezer, backs up into the freezer, and freezes.
You need to get to that rubber drain tube and clear it out.  But first you have to get rid of the ice that's blocking it.
There should be information on line as to how to do this for your particular model refrigerator.  On some you can access this from the freezer comparment.  But on others you need to in through the back of the refrig to get to the drain line.
Added Picture
Here is picture from https://www.applianceblog.com, that shows the condensate pan on a typical Samsung freezer, showing the ice buildup that results from a clogged drain tube.  The drain tube comes out in the center (usually) of the pan that's covered in ice in that picture.