See third paragraph here: Intro to Configurator
This applies for all side loaded equipment (Mac, iPhone, iPad). The 30 day opt out cannot be circumvented for side loaded devices.
Work with your sales channel to exchange Org ID and Reseller ID (you give them the Org ID and they give you the reseller ID - enter it in ABM) if you are buying from other than Apple. The reseller must participate in DEP program. If you buy direct from Apple you must work with the business team to get an Apple customer number. That will be added to your ABM and then all devices purchased through the business team will auto-assign to ABM. If you have the MDM token set properly, the devices will then auto-assign to your MDM.
You must stop purchasing via retail channels. If you do, then you will be side loading all of your purchases. It is a waste of time when it all can be automated. Doing it right means the device is assigned to ABM at time of purchase/build. By the time you have the physical device it is already assigned to ABM and scoped to your MDM for automated enrollment. Power on, choose language, country, accessibility, put on a network, enroll. It can be that simple.