If your trying to prime try taking the end of the outflow hose out of the water and pump again it should now prime. If not try flooding the intake hose with water so the hose is primed then try againe. But normally with any top motor basket filter you leave the out flow hose (leaving canister) end out of the water and in-flow hose (back to canister) in the water when pumping. Once primed return out-flow hose end back under water.
Magical! Previously, I would disconnect the inlet at the pump, and hold high then fill with water, and close the valve and place back in the tank. Reconnect inlet hose, close the outlet hose and open the inlet, pump a few times to pressurize slightly, then reopen the outlet hose. It worked, but seemed clumsy.
WIth CWO4GUNNER's suggestion, simply take the outlet hose out of water, with outlet below the water level. Pump and prime should cause the water to begin draining from the tank. While still flowing, stick the outlet back underwater. So simple. Done!