How Do I Reprogram My Keyless entry Transmitter??

[1999GTP1999]

When I bought my Grand Prix the previous owner had misplaced the transmitter, so I had to buy a new one. I've been trying a lot ways to reprogram it from information I'm finding on the net but none seem to work.... I read on a site that for the 1999 models only a GM dealer can reprogram them.... If anyone knows if that's true or knows how to reprogram it that would be great

Thanks