How to add missing Content-Length header to an HTTP POST request?

Have tried to send an APM HTTP Auth POST request to external authentication server which requires Content-Length header. Seems to be that APM HTTP Auth does not calculate and add the Content-Length header when sending a custom POST.

 

The POST content is small json data but its size varies. HTTP Auth sends the POST to a layered VS which converts the request to https, so can use iRules there. Tried to use HTTP::collect and then calculate the size from collected HTTP::payload and do HTTP::release. However it gets stuck..

 

Would be nice to be able to do it at the Layered VS. Alternatively thinking of using an iRule agent event in the VPE to form the json POST data and calculate the size into session variables prior the HTTP Auth box in the VPE and using them in the HTTP Auth custom POST definition.

 

Any advice?