Example binding to a URL tag from the chatwidget

$chan(window) chat tag bind URL <Enter> [list UrlEnter %W]
    $chan(window) chat tag bind URL <Leave> [list UrlLeave %W]
    $chan(window) chat tag bind URL <Button-1> [list UrlClick %W %x %y]
 
proc UrlEnter {w} {
    variable cursor:$w
    set cursor:$w [$w cget -cursor]
    $w configure -cursor hand2
}

proc UrlLeave {w} {
    variable cursor:$w
    if {![info exists cursor:$w]} {set cursor:$w {}}
    $w configure -cursor [set cursor:$w]
}

proc UrlClick {w x y} {
    set tags [$w tag names @$x,$y]
    if {[set ndx [lsearch -glob $tags URL-*]] != -1} {
        set url ""
        foreach {b e} [$w tag ranges [lindex $tags $ndx]] {
            append url [$w get $b $e]
        }
        if {[string length $url] > 0} {
            if {[catch {GotoURL $w $url} err]} {
                tk_messageBox -icon error -type ok \
                    -title [mc "An error occurred"]\
                    -message $err
            }
        }
    }
}