The UI#messagebox-class_method does not have input fields.
Perhaps you can writhe a code If the user cancel the dialog (false is returned) you will relaunch the same imputbox again… (it is not that elegant… 
)
Or you can use UI::HtmlDialog class to writhe your own “inputbox” method.
(BTW. here is a template: Define the width of a inputbox - #6 by tt_su )