Apple Script + VPN + Textbox
Here is a simple solution. 

tell application "System Events"
    tell current location of network preferences
        set VPNService to service "UJF"
        if exists VPNService then
            connect VPNService
        end if
    end tell
    delay 1
    keystroke "yourpassword"
end tell


If 1 second delay isn't enough for the popop to open feel free to increase it ! 

But don't forget, a password *should* NEVER be stored like that.