する verb and Transitive verb
やくそくする
「大きくなったらけっこんしよう。」「約束する？」「うん、約束するよ。」
"Let's get married when we grow up." "Promise?" "Yes. I promise."
ぜっ対にだれにも言わないって、約束してくれる？
Can you promise that you won't tell this to anybody else?
二人は、「指切拳万、嘘ついたら針千本呑ます」という歌を歌いながら指切りをし、日曜日に一緒にホットケーキを作ろうと約束した。
As they sang a song that goes, "If I lie, I'll cut my pinky off and drink a thousand needles," they pinky promised to make pancakes on Sunday together.
約束 is a “promise.” So when you add する to it you’re making it a verb, meaning to promise. Now promise you’ll remember!
The reading is the same as 約束 with する added. Make sure you know 約束 and you'll know this word as well.