Condition - Variable "sip_refer_to" we know it is refer method because it has a refer-to sip header in the "SIP:<user>@<host>" format. The regex below strips out the <user> and <host> and saves it to $1 and $2.
Actions - Use the custom action with the applicaiton application bridge. You can not use any of the bridge actions; they will not work. The data here is "sip/trunk/PBX_SIP_Trunk/$1" which sends an INVITE to the "PBX_SIP_Trunk" with the <user> part of the refer-to header. You will need to use your PBX SIP trunk name in place of "PBX_SIP_Trunk" It is required to use a custom action for all bridges with in the dial plan, in order for REFERs to be routed.
Once done click SAVE
Variable Name - sip_refer_to
Expression - ^.*sip:[\+]?(.+)@(.+)
Data - sip/trunk/PBX_SIP_Trunk/$1@$2
Variable - sip_force_full_to
Value - sip:$1
Data - sip/trunk/PBX/$1
3) Next edit any previous rules that bridged calls using the action "bridge/bridge to *" and replace them as shown below with the custom application. This It is required at this time in order for the REFER to work; this is something that may be resolved in future versions. Below is an example of how to bridge to the SIP trunk "PBX_SIP_Trunk".to use a custom action for all bridges with in the dial plan, in order for REFERs to be routed.
4) Once done Save, apply the configuration and test. The dial plan summary page should look as shown below.