The Healing Factor Mutation is probably the best mutation available in Fallout 76, so this is one of the ones you really want to get. When this is active on your character, you gain + 300 % health regeneration, and - 55 % chem effects, so it's incredible powerful for fast regeneration.
Like all mutations in Fallout 76, you get it from exposing yourself to radiation. The source of the radiation does not matter, but each 5 HP you lose from radiation damage, you have a 5 % chance to get mutated and get a random mutation. An easy way to get exposed is to find radioactive waste or drink unfiltered water.
There are currently 19 mutations in the game, so you will want to remove the ones you don't like, while trying to get the Healing Factor Mutation.
Once you get the Healing Factor Mutation, upgrade your Starched Genes Perk to Rank 2 to prevent Radaways from curing mutations, and well as preventing your character from being able to develop any new mutations.
Now you have the Healing Factor Mutation without any possibility of losing it or even gaining new mutations, so it's pretty much a permanent effect for you. Enjoy the incredible health regeneration!