To find your 16-digit activation key (6.4.x license code) please follow these steps below.

1. Open Citilabs License Manager

2. Wait for your license to display in the License Manager window

3. Right click on the license name (bold) -> click on "Porperties"

4. The 16-digit key is listed at the beginning under "AKey"

Note: selecting and using ctrl + c - ctrl + v allows to copy-paste