You can’t. Some dogs will do it and some won’t. It’s quite rare. They will have needed to retain their instinct to suck, while gaining their adult desire for sexual activity. Past a certain age, I believe they understand that sucking/licking a penis is sexual. Most dogs are happy to lick, but past a certain age do not retain their desire to suck.
You just accept and cherish whatever they are willing to do. Period. The most sensual, wonderful dog I’ve ever been with had no desire to suck, but he was a great licker (and kisser, and cuddler, and thruster…). And I can’t say that those dogs that do suck are necessarily more satisfying than those that lick. Though it certainly is an incredible experience.