Outlook appointments show a drop-down list for Location, populated with the most recently used locations added to appointments. You cannot populate this list with your own preferred locations, or delete a location from the list. You can, however, clear the list by closing Outlook, removing a value in the Windows registry and then restarting Outlook. Always make a backup of the registry before making any changes.
While you cannot populate the locations easily, you can add new locations and force older locations to drop off the list because the list is limited to the last 10 locations used. Create a new appointment and add a location then save it. Repeat for each additional location. This obviously works best if you want to replace older locations, not the last location you used.
To select a location from the most recently used locations, click the arrow on the right side to display the MRU list.
Delete LocationMRU Key
Press Windows Key + R to open the Run command and type regedit then press Enter.
Browse to the following path, right click on LocationMRU and select Delete.
In Outlook 2016, 2019, 365, the value is found at:
Do It For Me
If you prefer not to edit the registry yourself, you can use the appropriate .reg file for your version of Outlook. Simply download the file and run it (answering the prompts when they come up.)