$env:HTTP_HEADER_FIELD
$request:headers.HEADER_FIELD [3.4.4]
Such a construction will return the value of HTTP-header field, sent by browser to web-server (by HTTP protocol).
Example
^if(^env:HTTP_USER_AGENT.pos[MSIE] >= 0){
User is probably using MicroSoft Internet Explorer<br />
}
Names of HTTP-header fields are in uppercase letters and begin with HTTP_. All hyphens ('-') in these names are substituted by underscores ('_'). For additional information, please read your web-server documentation.