And if you get rejected with your first request, pester them with a follow up saying something like "Hey, I'm, a cloud customer. I don't control the whole IP range. I ask you to reconsider the initial rejection."

That's always worked for me with outlook. Takes a few days (which is why I say do it as soon as you know what your IP range is)

Be nice and polite (from what I understand real humans read that request) be honest with what you have done as of time of writing the requyest and what you are planning to do.

But be persistent with them and they will basically grant you a whitelist unless you start fucking up.