DHL Global Mail currently doesn't support Label References.
These will be available once techShip is updated to integrate to DHL GM API V4 (est date Sep 2020).
The current mapping works as follows:
- Reference 1 - Not operational
- Reference 2 - Not operational
- Reference 3 - Billing Reference 1
- Reference 4 - Billing Reference 2
From the V4 update on, the fields Reference 1 and 2 will allow to map label references.