Help. How to config outbound SIP blind transfer

I want to setup a blind transer for auto forwarding to outbound SIP peer.

I have context forwarding looks like this in extension.conf


exten => 511,1,Dial(SIP/sip_proxy-out)

This will do the re-invite, which is attendance transfer maybe.
But I want a blind transfer by REFER method. How can I do that?
I know that the transfer() function may be able to do that. But I don’t
know the syntax for that.
I tried
exten => 511,1,Transfer(SIP/sip_proxy-out)

So can any one give me a hint on this? Thanks!