You shouldn't flag them at all.
By flagging the post, you would essentially tell a moderator to handle the post. But there really isn't anything moderator-specific about it. Link-only answers are bad and they should either be:
downvoted
edited to include the information that was linked to
commented on
I usually use the auto comment:
Whilst this may theoretically answer the question, it would be preferableit would be preferable to include the essential parts of the answer here, and provide the link for reference.
Low quality flags should be reserved for posts that are beyond recovery and should be deleted.