Knocking at the gloryhole when you're about to cum isn't necessary. If I'm sucking your dick, I'm doing so with the intention of swallowing every creamy drop you offer. Most cocksuckers, men or women, feel the same way when we're at the gloryhole.
As far as attracting someone into the other booth with the intention of sucking them off, it's pretty simple. Do what makes you comfortable and puts you in the mood. I enter a booth, pop in some tokens and find a video that gets my juices flowing. I don't worry too much about trying to guess what might turn someone else on. As a guy looking to suck off guys, I do usually go with a gay or tranny vid though. When someone enters an adjacent booth, I show my interest by watching them through the gloryhole (I'm a voyeur, so watching them play with themselves and become aroused really gets me horny
). I make sure they can see me observing them by having my face and mouth close to the hole and then when they look my way, I simply insert my finger or fingers through the hole and tap the other side of the wall or I may beckon them towards me before withdrawing my hand and waiting for them to reward me. I don't think there is any right or wrong way to do this.
For the most part, if you're there to suck some dick and another guy is there to get his dick sucked and he doesn't care who does the sucking, you'll find each other. This isn't rocket science, just relax, let go and have fun.