If you bought a mortar truck, you can order it around using the HC control bar (default: ctrl-space). The mortar automatically deploys if it remains stationary for ~5 seconds. Once it is deployed, fire orders can be issued by selecting the mortar in the HC control bar and hitting shift-y/z, depending on your keyboard layout. Everything else is self-explanatory from there on.

Thanks for replying.

I was actually also wondering how structures in this mod work.
If we blow up a AAF factory, will they rebuild it or will they leave it destroyed?

They will rebuild it, and you'll get a notification once they are done. The current setup makes it a bit of an exploit to destroy factories/power plants/resources, so try to either keep those workers alive or take over the facility and repair it with your own funds.

Thanks for all the help, but my friends and I are facing a problem regarding town support.
We've been doing city supplies and got once city up to 20% support

However, for some reason we haven't recieved a city supplies mission for a long time. Evertime it's either destroy the ammo truck or bank heist. And now FIA support has gone back to 0 while AAF went to 100.

So how do we get those city supplies missions back?