Adding a pjsip header to your dial plan

How to add a pjsip header to your dial plan.

[extensions]
same => n,Dial(PJSIP/${EXTEN},60,b(add-alert,s,1))

[add-alert]
exten => s,1,Set(PJSIP_HEADER(add,Alert-Info)=hoge)
exten => s,n,Return