LTSC licensing is a bit different. You get one activation license key for your company, and you purchase stickers. These stickers will have a key but cannot be put into the image. It is only for tracking purposes. The key is good for 50,000 activations. When the system connects to the internet on first boot, activation will happen in teh back ground. This document might help: https://www.annabooks.com/Articles/Articles_IoT10/Windows-10-IoT-Licensing-Rev1.3.pdf and this article covers activation limitations: https://www.annabooks.com/Articles/Articles_IoT10/Windows-10-IoT-E-ActivationCheckVAMT-Rev1.3.pdf
You can work with your distributor to get access to the Microsoft Digital Operation Center. From there you can download the OEM version of Windows 10 IoT Enterprise LTSC ISO and get the test key that comes with the ISO. It is part of documentation that goes with the ISO. Or the distributor can get the documents for you. The test key can be put into the master answer file and you wont have to worry about losing an activation or be asked for a key. Put the real license key into the sysprep unattended file for images you are going to ship. Never give this key out.
Your distributor should have explained all this.