If you click the Help button on the Add Bot dialog there's a section that goes into more detail on Strategic Targets but the short of it is that you input the hex coordinates of the building. This isn't helpful on a random map, but if you're using a premade board you can look at it in the Map Editor to get the right hex number.
If you do want to work with a random map, you can generate the random map in the Map Editor, note down the proper coordinates, and save it and then load that map in the MegaMek lobby for your game.