Adding weedkiller (Sodium Chlorate) to the area you've cleaned is in my opinion a bit of a gimmick that might help you get one or two customers.

I can tell you that it's unlikely those pesky weeds will be stopped for very long, so I'd advise you not to oversell the benefits of your weedkiller application service.

Sodium Chlorate is usually bought in crystal or powder form. You put some of it in a watering can with a fine rose spray, add some water, stir the crystals allowing them to dissolve, and then pour it over the area you've pressure washed. For the sodium chlorate to be effective it needs to be done when the surface is dry and where the weather is expected to be dry for the next couple of days.