I recently migrated my own custom email domains away from GSuite to Office 365 Business Premium. I needed tech support for a few things and found that I could submit a support case through the Microsoft 365 admin center. The case creation was super easy, response time was quick, and the help was primarily email based. The tech called me by phone the next day to see if my issue was resolved and we got to talk about it more. From https://portal.office365.com, login to your Office 365 with an account that has Admin access. From Admin Center, click the Question Mark in the top right corner. Then click on the red Need Help? button at the bottom of that window pane. It will step you through the knowledge base of potential solutions and then it will offer options to submit a service request by phone or by email.