How to make a field required

I'm using Adobe LiveCycle 8.0. I just created my first form. There is a few fields I would like to make required. I already have the TAB order the way I need it to be. Here's what I need to happen: If a user starts filling out the form & does not fill in any information in the client ID field, & they hit the TAb key, I want some kind of error to pop up telling them the client ID field is required. Is this possible? Thanks