Hi there! You absolutely do need a preservative, so good job for recognizing that.
You can use Liquid Germall Plus at .5%, or Optiphen Plus at 1%.
You didn't ask about this, so I hope you don't mind some feedback about the recipe itself. I am guessing your intent is that the beeswax will serve as an emulsifier to keep the water and oils from separating after you mix them. Unfortunately, beeswax alone really doesn't emulsify anything unless it is combined with borax.
While some mommy blogs claim success with just beeswax, I haven't personally been able to replicate their results unless I keep the product refrigerated and use it up very quickly. Have you considered using e-wax instead of beeswax?
As a final point, the generally accepted usage for the term "body butter" is an anhydrous product, i.e., one that doesn't contain any water, and therefore, no preservative. Once you add water, an emulsifier, and a preservative, you are really talking about a thick lotion or cream. You could call it "emulsified body butter," which does at least clarify that the product contains water, and thus, a preservative.
Please know that I'm honestly not trying to be the word police here. It's mostly that some folks have very strong feelings about not using any preservatives on their skin, so it's one reason for keeping the terms straight. The other reasons include general clarity about what kind of product is being discussed, or what search terms to use when seeking information on the Internet. HTH!