Regardless, it's a really, really bad idea. Here's why.
1. Catheters hurt. Like a bitch. Even when done by a medical professional
2. This is not medical grade liquid morphine and you'd be want to be very careful about extraction, which you probably can't do properly outside of a medical setting.
3. You're just begging for an infection or something worse doing this yourself and the effects might be pleasant but could also include tissue damage, bladder and kidney infection, and renal failure.
I've never even heard of this method, but please don't try it. If you really have the urge to put this somewhere other than the normal places you might administer morphine, just plug the stuff (use it rectally). It will work better with far less of a chance of complications.