I seem to have only 1/2 the wristband in the box

What watch did you you exactly? With what band?

Some bands are a single piece. For example the following watch comes with a Solo Band which is a single piece that attaches to both sides of the watch: Apple Watch Series 7 Green Aluminum + Green Solo Loop - Apple.com

Go ahead and look through the full contents of the box. If there really is only one half the band, take it to your Apple Store with your receipt and Apple will make it right for you.