We asked an 886 partner this question:
Never ask for an introduction — ask if someone would be willing to forward something like an email. Then provide the introducer with an email they can forward that explains what you are doing and why you are trying to make contact with the target person. That person can then just forward the email and ask if they'd be interested in an introduction. Basically the person making the introduction needs to make sure it's ok with the other person first, so make it as easy as possible for them to do that step.
