I have built a long range trike and It has two cargo compartments on the side. I can’t see any way to put items into these compartments though. How do you interact with the trike to load cargo onto the bike from your back?
The long range trike does not allow this. Those cargo boxes you mention on the side are actually batteries. This is what gives the bike the extended battery range. The cost of this extended range comes at the cost of the cargo space.
You are unable to load cargo onto this type of bike. Use a different type and you will see a prompt to hold square to open the cargo menu. Having the bike nearby will also allow you load things onto it from the terminal menu after you have accepted jobs.