Mutations in Fallout 76 can be very powerful, but obtaining them can be a hassle since you always get a random of the 19 available mutations when getting exposed to radiation. However, it is possible to get a specific mutation, making the entire grind to get the correct mutations a lot easier!
In order to get specific mutations, you will need to find a Mutation Serum. These serums can be used by your character to inflect it with the mutation you desired. There are basically two ways to get the Mutation serums:
1) Buy it from the Enclave Science Wing MODUS Vendor. This vendor sells all the mutation serums for 5,000 caps per serum. This is obviously very expensive, but it can be worth it if you only need one or a few mutations.
You will need to have progressed pretty far into the Enclave quest chain before this vendor is available to you.
2) Buy it from another player. There are also recipes to crafting the Mutation Serums, and many players sell these for far cheaper than the MODUS Vendor. So look for traders on a forum, and you might be able to snatch the serum you desire for far cheaper than what it would cost from the vendor in the Enclave Science Wing.
And that's pretty much it. No one has reported that these serums can drop from enemies, so you won't be able to farm them right now. But there are many ways to farm caps easily, so that's a good option if you don't have the funds to buy the serums just yet.