A friend was visiting the other day. She's the mother of an infant and has no experience with potty-training. My Ami (2 1/4) woke up from his nap, during which he still wears diapers. I took his diaper off and after 20 minutes or so I told him it was time to go to the potty. (He's still at the stage where if I don't remind him, he's likely to have an accident.)
Ami is cranky when he wakes up from his nap. He wasn't having any of it. He refused.
My friend asked "How can you make a kid go to the potty if he doesn't have to [or more likely just doesn't want to] go?"
I went and got the kitchen timer. I set it for two minutes. I said "Ami, when the timer rings, we're going potty."
He was not impressed.
I tried to distract him. I said "Listen, the timer's going tick-tick-tick-tick-tick-tick-tick." (Usually he loves that.)
He said "Timer NOT go tick-tick-tick."
I walked away.
When the timer rang, Ami slid out of his booster seat and dashed to the bathroom!
When we're at the mall, I do the same thing, except I set the alarm on my phone and I let him hold it till it rings.
If for some reason this doesn't work, you can also use the timer trick another way. Set it for one minute and say "Let's just sit on the potty until the timer rings and then we can go play." He might squirm or even cry, but if you can keep him there for a minute, chances are he'll do his business while he's waiting for the timer to ring.
I can't promise this will work with every kid, but it works with mine.