Hi! I'm trying to help a campaign member of mine who wanted to make a custom race (Half-Daelkyr), and I was trying to use the custom race feature here. I'm almost done, but for one of the racial features I wanted to add the cantrip Mind Sliver and Dissonant Whispers as a once per long rest spell. This is my first time making a homebrew race, and when I tried to add the spells, both were not on the list I was given. Do I need to go to advanced spells?

You might want to try the Homebrew forum. They might be better equipped to help you out. Sounds like a cool idea though 

You need to own the source in order to include something in homebrew. Either the book the spells are in, or just the spells themselves.