Do you know if you want Rimguard, recommended or Calcium Chloride, not recommended because of the corrosiveness in that. Both are 30% heavier than windshield washer fluid. But all will work.
Do you have a big valve stem in the rear tires that will facilitate pumping fluid or is it a regular stem like the front tire or like on car.. Are they steel valve-stems vs rubber. I am assuming they are steel. They need to be steel, or maybe brass.
The following description worked for me;
You would need a small electric pump to pump the liquid from a barrel through a valve stem.
Jack the tractor up on one side just far enough with the tire just putting pressure on the ground with the valve stem in the 12:00 position. This should support some of the weight of the fluid when you begin filling.
Let the air out and when the air is out just begin pumping the fluid in using the adapter dex3361 suggested.
When the fluid starts coming out of the relief in the adapter the fluid is the right height. Then shut off pump and disconnect it from the tire and install valve core. Fill tire with air to rated capacity and you should be good to go.
There are cases where the bead broke loose as soon as the air was out of the tire and fluid began being pumped in. If I remember the cause was because the tire was off the ground when fluid was pumped and the weight pulled the bead loose.
A few miles down the road from me there is a big farm operation where they have you drive the tractor over and they fill it next to their big RimGuard tank, but I cheaped out and filled mine myself.