Hello, welcome to JustAnswer.
What is the serial number on that motor?
To fill the reservoir you need to tilt the motor all the way up, then unscrew the filler cap. (it has a hex head with a slot in it) Fill it all the way up until the fluid reaches the threads for the cap.Replace the cap securely and lower the motor all the way. Cycle the motor thru it's full tilt/trim range several times and repeat the fill process until it no longer needs any oil to bring the level up.
Do not trim the motor with that cap not securely in place.