Hi Lewis,
Good question, thanks for raising it on the community! Our guide on Best before dates (BBE - product expiry dates) explains how to set this, but the answer is to set a best before from delivery on the beer itself. As long as you have done this and have set the Label best before display option in Settings → Delivery settings → Delivery label/document settings to “Display the best before end date on the label if possible”. You should be set.
Let us know if you have any questions at all!