DeploymentPro @Destination Domain Name

We're migrating GSuite to Office 365 using the User Migration Bundle Package. Gsuite is currently setup with companydomainname.com and I've setup the O365 tenant as companydomainname.onmicrosoft.com. During the DeploymentPro Configuration the system is requesting the @DestinationDomainName and my inclination is to use the current onmicrosoft.com domian address because in the destination endpoint I do not have the vanity domain described anywhere at this point.

If my inclination is correct, do I proceed with the Deployment Pro configuration and let everything sort itself out when I move the MX record. Or

Should I add in the vanity domain now to the Office 365 tenant as an alias and just allow it to lie dormant until I can make the MX record switch. I'm a little confused at this point and can't seem to locate what I'm asking in any other documentation.

Thanks

Jeff Adzima