When you set up an Office 365 subscription the information that you supply for the initial setup is critical going forward. The reason for this is your “permanent” internal O365 identity will be created as “something.onmicrosoft.com”. So, if you set up as “mytrial.onmicrosoft.com” and you decide to convert from a trial to a “live” subscription you’ll be stuck with mytrial.onmicrosoft.com as the identity cannot be changed after the fact.
So, I suggest you try for an ID that is as close as possible to what you want going forward and skip having words like “trial” in the name. Keep in mind that this is the “internal” email domain for O365, your “real” email domain/domains will be overlaid on top of this primary ID as you go forward with O365 so you are not changing your email domain. But, we have seen many customers that are frustrated because they really didn’t want “mytrial” (or something like it) going forward.
So, give it a little bit of thought up front and save the angst!