I have a combobox on a form, how do I go about making it so when the user enters something that is not a valid selection in the combobox that a popup appears telling the user that the entry is not valid?
However, do you know a way to make a custom popup for the situation I am referring to?
Yes, I believe the standard way of doing so would be to create a small form and design it the way you want, and use that as your pop-up. You can then set the pop-up and modal (other windows are disabled until you are done with your popup) properties to yes. Also, you will want to change your form border style to dialog.