Are you short any transmission fluid? When did you have your transmission filter and fluid changed. It should be changed every 25,000 miles or so (check with your owners manual.) To answer your question. Pull out the dip stick that you check you transmission fluid and pour it down the same tube. You are on your own
In the same place you add the fluid, through the dip stick tube.
For vehicles hauling loads or doing a lot of long hours of driving...there are separate cooling "radiators" and lines available at dealerships or auto parts stores. They usually are easily installed or can be by shops. Stop by & talk to an automatic transmission repair shop for information.