You can find the value (formatted like prod-aaa-bbb-v0.0-000a000a0a00) under the JMS Adapter for BizTalk Registration Tools Offline License Activation Tab.
Access the tool via:
Start → All Programs → JMS Adapter for BizTalk version → JMS Adapter for BizTalk Registration Tool (.NET version you wish to support).
Once the tool has launched, select the Offline License Activation tab and return the Registration Key value.