Edit for context:
My view is transracial isn’t valid and this person is trying to dogwhistle. I’ve already blocked this person, and now they’re going after my friend saying my friend is transphobic because they disagreed with them about transracial being a thing (they’re purposefully leaving the context out so my friend looks transphobic when what my friend really said was transgender is valid but transracial isn’t)
The best way is asking: what’s your point? Is it that transgender shouldn’t be accepted or that transrace should?. And proceed from there to either defend transgenderism or criticize transracism accordingly.
First let me start by saying I strongly dislike the race therminology, but I’ll use it here for consistency, although normally I would call it ethnicity.
The difference between those lies in that gender is a social construct, and race is not. Race has some biological meaning, just like sex, people can’t change their sex (yet), they can’t change their race (yet).
Gender is a social construct, it’s things that have nothing to do with biology but that we as a society attribute in general to a specific sex. A similar concept for race would be culture, a person can be of the sex male but prefer to wear clothes usually associated with female sex, just like someone can be of the white ethnicity but prefer to hear music usually associated with black ethnicity. I wouldn’t call Eminem or Michael Jackson transrace, what would that even mean?