I'm homebrewing a race off of WotC materials, and one of the racial traits goes as follows:

"You know one cantrip of your choice from the druid spell list. Wisdom is your spellcasting ability for it."

How do I add this choice to the race I'm homebrewing?

You will want to add a "Spell" to the feature. In there you can add "Druid" to the spell class field, and set the spell levels to "Cantrip". You'll also want to set the ability score to Wisdom.

Won't that add all the cantrip level spells from the druid list? How do I make sure I'm only adding one?

No, it will give the player a choice of which spell to add based on the parameters you set. Give it a try.