Applescript refer a link to a file in current finder window
This seems to work for me, 

I'm setting the patch with the variable P and set a to POSIX file p

Then I create a dialog with the default location of the variable A

set p to "/Users/usernmaHere/Desktop/Time"
set a to POSIX file p


set config_file to choose file with prompt "Locate your .plist config file and click OK" of type "plist" default location a


I added file limitation with " type "plist"