Transfer Calls between ATA Ext and Deskphone


I am trying to figure out how I can transfer calls from an Analog Extension (provided by a Grandstream HT802) to a regular desk phone (such as a gxp1608) when both are connected to FusionPBX. I have been through the settings on the HT802 to see if there is any functionality that I can do with the flash key on the analog phone, but I'm not having any luck getting this to work.

Any help would be greatly appreciated.