Most tank additives recommend pouring the additive into a low tank and then filling it to get some mixing from the nozzle. I add it to the jerry can and then fill from the barrel. Then I dump the jerry can into the tractor tank. If I was fortunate enough to be able to fill directly from the barrel then I would add the PS to the barrel before filling it at the fuel depot. Least ideal would be additive in the tank but I think we are splitting hairs.
Most diesels use a pump through type fuel system where fuel that doesn't get squirted into the engine is circulated back to the tank so the tank gets stirred up pretty good.