You can't create masks inside Animator at this point, but here are a couple workarounds:
You can simulate a mask by creating a full-stage layer that's the same color as your expected (host codebase, embedding) background, but with a hole punched out. You can then animate elements underneath, executing a mask.
This doesn't work for every scenario, but sometimes it's good enough.
You can create your mask in Sketch and mark the group that contains the mask as exportable, and it should reflect correctly in Animator.