Gtk.Label.prototype.select_region

function select_region(start_offset:Number, end_offset:Number):void {
    // Gjs wrapper for gtk_label_select_region()
}

Selects a range of characters in the label, if the label is selectable. See Gtk.set_selectable. If the label is not selectable, this function has no effect. If start_offset or end_offset are -1, then the end of the label will be substituted.

start_offset: start offset (in characters not bytes)
end_offset: end offset (in characters not bytes)