The Sentinel HQ you are sent to after selecting your character is chosen at random and cannot be manually selected. If you wish to play at a specific Sentinel HQ, follow the steps below after loading into a Sentinel HQ.
1. Open the Options menu.
2. Go to the Community tab.
3. Enter the code for the Sentinel HQ you wish to go to in the "HQ Codes" option.
You can confirm the HQ code of the current Sentinel HQ you are in from the top right of the Community tab.