If you're fully setup an a MOSPA then you can login to O365 and go to the "Partner" area. In there, you can create a purchase offer for the type of licenses and that generates a link. From there, the customer (or you can do it) uses the link to actually sign-up for O365 services.

You'll get the Partner of Record (PoR) fees by being the Partner of Record (even if you don't want them). The key here is that your Partner ID is listed as of the PoR for the account. This associated them in your Partner portal which then gives you a 1-click way to get logged into their O365 org as an admin to manage their O365 account. That's the thing being the PoR gets you from an admin standpoint, and you'll get the money too.

You might as well have them purchase through the links you provide vs. they add you later. This way you get the 12% on the purchase, as opposed to just the 6% on the recurring fees if they add you later. The end result from a management standpoint is the same.

Beyond this, I also create an actual user for my company in each customers O365 org. It's a non-licensed user but it's a full admin. The main reason I do this is because you can't user your access through your Partner center to purchase additional licenses for the customer, but you can if you have the actual admin account setup under their org.