This video shows how to connect your DEP account to SchoolMDM. The video assumes your DEP account is configured as part of Apple School Manager, although the process is similar even for non-ASM dep accounts. Once DEP and VPP are configured in Control Tower, rosters and users will automatically sync from ASM to SchoolMDM.
Note: This video references TabPilot, the prior name of TechPilot
Update: Near the end of the video, a reference is made to the "DEP Device Lookup button" to check that devices are assigned. The button has been replaced with a tab labeled "DEP Devices" that serves the same purpose.