Stare at the buried tank inside the hole until it glows and shakes. It will then be permanently available in your Legacy faction.
For a step-by-step visual on the file-swapping method and specific IDs for the tank gun and projectiles, watch this guide: Make your own Tank - TABS Yukishi Goemon YouTube• Apr 30, 2022 Guide :: How to make Tanks,Raptors,and Horses how-to-make-a-tank-in-tabs-unit-creator
Open the .unit file using Notepad or Notepad++ . Find the line "m_unitBase" and change its "m_ID" to 1620026876 (the internal ID for the Tank base). Stare at the buried tank inside the hole