How to salt whole potatoes before roasting or cooking in the microwave?
You cannot season a whole potato from the outside when baking or microwaving, the flavor may penetrate a small amount but most of the potato will be unseasoned. Boiling potatoes in salty water will give you a better result, however the skin of a whole potato will still block much of the flavor. 

The only way I could think of to salt the inside of a whole potato would be so inject a salt solution into it with a syringe, which seems excessive. You'd probably be better off simply cutting the potato open after it's cooked and seasoning it then.