Is the Trimble doing steering or just doing section and rate control on the sprayer?    If its not used for steering its probably not worth spending the $'s on RTK.

If the Trimble is doing steering and you want RTK in it, you will first need to get it unlocked to RTK before you can do anything else.    Make sure you're sitting down.




If your 750 is currently only WAAS, you need to buy 2 unlocks.  First you need to buy the unlock for Omnistar, then you have to buy the unlock for RTK.    They each cost ~$2200, you have to buy both.

Most of the time, you can cheaper buy a used screen that is already unlocked to RTK than spend the money on unlocks for your screen.
 
After you have an unlocked screen, then you need to get a RTK correctional signal from somewhere whether thats from a CORS network or a private subscription service like Digifarm.     Digifarm subscriptions are somewhere in the neighborhood of $800-900 per screen per year.    If your state has a CORS network that is free that may be a viable option but does not come with any tech support, you'll be on your own to get things setup/working.