[squid-users] How to force squid to ask for client certificate during tls handshake on https_port?

[Pavel Kazlenka]

Hi gentlemen,

I have https_port configured as the next:
https_port 3128 cert=/home/tester/certificates/server.crt 
key=/home/tester/certificates/server.key

and would like to force squid to retrieve client's certificate. 
According to 
http://www-01.ibm.com/support/knowledgecenter/api/content/nl/en-us/SSFKSJ_7.1.0/com.ibm.mq.doc/sy10660a.gif 
, client certificate request is optional and looks like squid doesn't 
request the one by default.
Squid version if 3.5.1.

Is that possible at all and if so, how to do this?

TIA,
Pavel