Code:
#include <amxmodx>
#define STEAM_GROUP_URL "www.steamcommunity.com/groups/mygroupusername"
public plugin_init()
register_clcmd("join", "hfreflfy");
public hfreflfy(id)
show_motd(id, STEAM_GROUP_URL);
Not 100% sure if it works out with steam browser protocol, but you may give a try.
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