Thanks for link but it seems like it is not required.
Here is my send string "OPTIONS icap://avscanner/avscan ICAP/1.0\r\n\r\n" and managed to get a response.
***** WORKING CAPTURE *****
OPTIONS icap://avscanner/avscan ICAP/1.0
ICAP/1.0 200 OK
Date: Tue, 20 Feb 2018 07:16:34 GMT
Methods: RESPMOD, REQMOD
Service: ProxyAV AV scanner 3.5.4.1(182496)
ISTag: "1519109626"
Service-ID: avscanner
Encapsulated: null-body=0
Max-Connections: 100
X-Include: X-Server-IP, X-Client-IP
Allow: 204
X-Scan-Progress-Interval: 3
X-Abandon-Supported: 1
***** WORKING CAPTURE *****