I think the issue is in the way a standard 'inputbox' handles the 'values'.
The last value always has focus, unless there are 'lists' in the values, then the last value that has a list has focus - irrespective of where it is !
It's a limitation of the standard API interface.
The 'Tab' will key step through the various values [and the two buttons] in turn starting at the first value, if that would be quicker than clicking the mouse in a dialog itself...
If a list has focus then typing the first letter of the desired item [the list jumps to that item] and/or using the up/down 'Arrow-keys' to step through the list from the current item can be useful if the list is too long to be fully displayed on screen in one go !