Here's one way as an example for Keepsake Bay.
Launch the game, load Keepsake Bay in quickmatch with two factions. After loading, in Start new game menu, select Advanced preset and click on Open match settings folder. Create a file called map2.xml there with e.g. notepad with the following contents:
Code:
<commands>
<command
class="start_game"
vehicles="1"
max_soldiers="100"
player_ai_compensation="0"
xp_multiplier="1.0"
rp_multiplier="1.0"
initial_rp="500"
initial_xp="0.5"
friendly_fire="0"
base_capture_system="any">
<faction ai_accuracy="0.95">
<base key="shop lane" />
</faction>
<faction ai_accuracy="0.95" />
</command>
</commands>
Then go back to game, type map2.xml for Match settings filename and hit Start.
Base keys are most of the time base names you see in the map view, but there can be exceptions. You can basically open up the relevant map file, e.g. C:\Program Files (x86)\Steam\steamapps\common\RunningWithRifles\media\packages\vanilla\maps\map2\objects.svg in Inkscape or some text editor and try to locate the base you have trouble starting from. With a text editor, if you'd search for
bases layer, you'd see various rects with name and a key field there.
Here's some more parameters that can be supplied in start_game command
https://runningwithrifles.gamepedia.com ... start_game