why would you want to use an input box? Why not a straightforward ListBox which you can manage to only select one value if you wish?
An InputBox is associated to a variable. You can limit what values the varaiable can take by settings "Constraints" in the properties dialogue box.
If your dates are in a field in your data model and you would like to constrain the variable to having to select from these possible dates then on the "contsraints" tab click on the option "predefined Values in Drop-down", click on "listed Values" and enter an expression such as this