Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
This is the generic detection for Trojans that continue to change the start page of a browser. The start page is a site, or part of a set of sites, that a browser opens when it is run.
Malwarebytes can remove Trojan.StartPage without further user interaction. After removal, you may want to change the start page back to how it was before the infection.