Add an ALLOW header to INVITE

What's up Acme community, this is my first post and I apologize for being a complete HMR noob.

 

I am interested in adding an ALLOW header to outbound INVITEs leaving my network. I understand some basics on creating header-rules but I was hoping for some help and theory behind seting the "new-value" parameter.

 

Here is what I have so far:

 

header-rule
                name                           add_ALLOW
                header-name                    ALLOW
                action                         add
                comparison-type                case-sensitive
                msg-type                       request
                methods                        INVITE
                match-value                    
                new-value                      

 

I have reviewed the HMR developers guide and saw the built-in variables, but since ALLOW is not one of them, and I would like to understand