How to make new customers appear in drop-down menu?

SUGGESTED

When we had Sage installed, all of our customers were imported into the system. When I start to type in the 'Customer No.' field, a drop-down menu appears with predictions of those customers. However, when I add a new customer, I have to click the magnifying glass if I can't remember the full customer # because they don't show up in the drop-down. Is there a way to update this? See below.

Thanks in advance for help!!