Hmm, I'm going to assume from your post that you have messed your diaper in the past, but only been able to do so with laxatives, and you don't want to have to use laxatives to poop in your diaper? If that's correct, then I'm going to assume you're trying to do it from a sitting position (like your on the toilet, but probably on your computer chair). Unlike a toilet, which has only free space under your bum, sitting on a diaper has loads of compression acting against your bum, so it'll be difficult to mess that way, unless your poo is quite soft/runny.
I see other people saying to "hide in a corner like a toddler" to do your business, but they're forgetting the most important thing: squatting. Squatting out in the open is how our species pooped for thousands of years before we developed the outhouse/flush toilet, and it's how our bowels are accustomed to be able to easily pass poo. If you're having trouble pooping your diaper, and you're not using a squatting position, then try that, and see how it works for you.