The transer case has a

Replace the leaking seal...

There is no "easy" fix - if you have a leak then something isn't right...

Clean it thoroughly and then run it a few kms and see if you can tell where the fluid is leaking from. At that point you will have a better idea of what needs fixing.

Placing sealant on the outside is unlikely to resolve the problem. But if you must take that approach then you need to thoroughly clean the area with solvent (meths will work if you have nothing else) to remove all grease or fluid residue, give it a light sand to provide better adhesion and use a high temp silicone applied liberally. Leave it for 24 hours minimum to cure and if you are lucky it will hold.

You could also try an epoxy glue but these tend to soften under heat so if the transfer case warms up enough it may void the seal. I have however repaired a hole in a transfer case with Epoxy Steel putty and the repair held as good as a weld, the main thing is to make sure anything you use has a good clean surface to stick to.

Good luck.