Collect the user input in a global field. Convert it into a variable then if you want. Otherwise you can treat it as having all of the characteristics of the variable you wanted.
And if you are using FileMaker 11 or earlier, keep in mind that button 1 is the only button that commits user input in input fields.
Also, there are many times when show custom dialog just doesn't cut it as a dialog for getting user input. In such situations, it is possible to use new window to open up a window for input that includes all your typical layout features--including value lists and graphic elements.