when i click it the field does not get the focus. i can access the field via pressing the "tab-key"
It sounds like you've cancelled the default action for the mousedown event. Search through your HTML and JS for onmousedown handlers and look for a line that reads.
return false;
This line may be stopping you from focusing by clicking.
Re: your comment, I'm assuming you can't edit the code that adds this handler? If you can, the simplest solution is to just remove the return false; statement.
is there a way to just add functionality to the event-trigger by not overwriting it?
That depends on how the handler is attached. If it's attached using the traditional registration method, e.g. element.onmousedown, then you could create a wrapper for it:
var oldFunc = element.onmousedown;
element.onmousedown = function (evt) {
oldFunc.call(this, evt || window.event);
}
Since this "wrapper" doesn't return false, it will not cancel the default action (focusing) for the element. If your event is attached using an advanced registration method, such as addEventListener or attachEvent then you could only remove the event handler using the function name/reference and reattach it with a wrapped function similar to the above. If it's an anonymous function that's added and you can't get a reference to it, then the only solution would be to attach another event handler and focus the element manually using the element.focus() method.