I have a form where a dropdown list is dynamically build based upon previous input.
Next to this list, there is a button which opens a popup window where the user can choose some other items that are not displayed in the list.

I want the user to look at the options in the dropdown list first before he/she pushes the button to choose some other option.

How should I start with this one? How could I use a alert message when the user clicks the button without looking at the list first?

Thank you for any suggestions!