Does the spreader have a moniter or control of some type that goes in the cab?
I have a friend that has a hyd driven spreader but it's driven by the tractor pto. The spreader itself has it's own hyd system that does all the work. The speed of the slingers and chain can be changed but it's done from the ground.
Unless it has something I don't know about the tractor is not equipped with any type of speed / flow control from the cab. A flow control rear remote is a option that can be added but it adjust at the remote from the rear of the tractor. If you have gorilla arms you might reach it from the cab but other than that I don't think it can be done.
If it really is driven by the tractor remote you will need the flow control remote or it will overheat the hyd system.