To assign a specific drone (UAV) to a legion, you must fully upgrade its energy conversion, which can be done by following these steps:

-Go to the Airport and click on it.

-Select UAV.

-Click on the "Energy Conversion" at the top right.

-Choose the desired drone (UAV) from the tabs and click on it.

-Upgrade all of its skills to the maximum level.

-Once fully upgraded, the drone (UAV) will be ready to be used in the legion.