user agent is software (a software agent) that is acting on behalf of a user. One common use of the term refers to a web browser telling a web site information about the browser and operating system. This allows the web site to customize content for the capabilities of a particular device, but also raises privacy issues.
There are other uses of the term "user agent". For example, an email reader is a mail user agent. In many cases, a user agent acts as a client in a network protocol used in communications within a client–server distributed computing system. In particular, the Hypertext Transfer Protocol (HTTP) identifies the client software originating the request, using a "User-Agent" header, even when the client is not operated by a user. The Session Initiation Protocol (SIP) protocol (based on HTTP) followed this usage. In the SIP, the term user agent refers to both end points of a communications session.
To solve this challenge you need to manipulate browsing the page by modifying your browser user agent. a correct user agen that carried user name in it will solve the challenge .
Change your broswer user agent to (admin) to get the password .