If your dinner jacket was constructed without a lapel buttonhole and you wish your tailor to add one so that you may sport a boutonniere (which the British confusingly also call a “buttonhole”), you have a couple of options. You can get a keyhole shaped opening like the ones used for the jacket’s waist and sleeve buttons or you can get a straight hole like the one shown here.
Some will argue that the latter is a mark of a better suit but the fact is that both styles are used by quality suitmakers. Therefore it’s a matter of personal preference. I’m partial to the inconspicuousness of the straight option given the very visible placement of the hole high up on the chest and (usually) surrounded by satin.
Also, whether your buttonhole is original or “after market” (as automotive enthusiasts would say), consider having a thread added behind the lapel to hold the flower in place. A more fussy alternative is a minature boutonniere holder, a small metal vial that clips onto the back of the lapel and provides water to keep the flower fresh. Unless you’re planning to wear the flower all week long, it seems a bit over the top.