Welcome to JUST ANSWER small engines, let me try to help you !!
I find a T18 model.?? But with any of the trimmers, can you see where the line goes into the tank? With new line in hand and old line removed from the grommet , if it has grommett, the new line has to be inserted through the tank grommet or the tank wall. You do this by using sharp blade, and make a point on the new hose my cutting it at a long angle. With a good long point to use as a guide, you start the hose into the tank and then use needle nose or forceps to reach in and pull the hose through to the tank.
Pull enough hose through to cut off the end square, and attach the weighted filter, then pull hose back through until length is correct and attach it to the carburetor or primer.
Your primer is part of carburetor, but basic principle is the same.