To buttonhole someone
Solution 1:
It's usually used in connection with someone with the proverbial bee in the bonnet, or someone with particularly urgent business. "I was buttonholed by a bible-thumper in the street". "She buttonholed him outside his office and demanded a decision". You wouldn't use it to describe a boy trying to get a girl's attention. You might say, "he came on to her" or "he got alongside her" or (in the north of England) "he tried to get his feet under the table".