In the old days, when the young men would ask a girl for dance, she would write her name down on a piece of paper. The young pursuiter would keep the paper and present it to her when he wanted to dance with her.
To be the obvious choice, a perfect fit.
ADDEDUM: Generally spelled shoo-in. A sure thing. One who is bound to be successful. A certain and easy winner, as among candidates for an office or...