Not sure why there is not a Validation tab associated with Combo Boxes, but there's not.

Go to the properties of the combobox and select All Properties tab.

Expand the data section. For the "validators" property, click to add a validator and choose validateRequired.

Type in the message.

That should be it...

It doesn't work "as is".  I did get more success by adding the dijit.form.Combobox property, THEN the validateRequired works mostly as expected.  But now I want to check it before it's saved - using currentDocument.getItemValueString just gets the back-end value.  getComponent doesn't for me, either.

So, what's the best way, in a combobox field (I could also do this with a button if necessary) to get the front-end value of a field for validation?