Based on your title, it sounds like you did not buy these through Business Sales or a DEP authorized reseller. You bought them through a retail channel, thus negating chain of custody offered by ABM and the right sales channel. If so, then you will need to user Apple Configurator to side load the devices into ABM. You should do this BEFORE you start using the devices as if you set them up, you will need to wipe them in order to side load them.
The easiest way to do this is with an existing iPhone and Apple Configurator. The reason is that the iPhone version of Configurator will allow you to assign the device to a specific MDM at time of capture and you need no cables, don't need to learn how to enter DFU mode, etc. Simply have the capturing iPhone in proximity of the device running Apple Configurator.
Apple has some decent resources available here and here.
As you look to purchase new devices, make sure you start with purchasing them properly. If you are working direct with an Apple Store, talk to the business sales team and inform them that you already have ABM and all hardware needs to be linked. If you are working with an Apple Reseller, provide them with your Org ID and ask them for their Reseller ID. Enter their Reseller ID in your ABM and this will allow them to associate new hardware with your ABM tenant.
Hope this is helpful.