I am adding a custom http header but how to print it out using irules?

Here's what I have done using curl -

The curl request is seen with "-v" it shows that the devicetyp header is being added to the request, but how do i fetch it from irules?

I can see the header if i use this irule with a loop:

when HTTP_REQUEST { 
foreach aHeader [HTTP::header names] { 
log local0. "HTTP Request Headers: $aHeader: [HTTP::header value $aHeader]" 
} }

I want to grab this header value and store in a customer apm variable like session.custom.device.type.. i understand there are other ways to do this but we have chosen this for now.. any ideas??