Adding devices to Autopilot requires gathering the hardware ID hash of the device. If you are buying new devices from a supplier they can usually provide the details in a file that can be imported into Autopilot.
If you have existing machines that you want to enable for Autopilot deployment, Microsoft have provided a script that will gather the correct information and create a CSV file that can be imported into Autopilot.
md c:\\HWID Set-Location c:\\HWID Set-ExecutionPolicy -Scope Process -ExecutionPolicy Unrestricted Install-Script -Name Get-WindowsAutoPilotInfo Get-WindowsAutoPilotInfo.ps1 -OutputFile AutoPilotHWID.csv
The above script will need to be run on each device that needs to be added to Autopilot.