Way of remembering "enhavas": "en"=in, "havas"=has. So, putting the two together you have "in have"
Well, that's not simply a mnemonic, that's actually how that word came to be.
Would "Her coffee has milk in it" also be accepted? I know it's the same meaning, but I'm curious if it'd be correct.
I tried "Her coffee has milk in." and that wasn't accepted. Probably too slanglike.
As for your version, it probably should be accepted in my opinion.