I would imagine that you would need to add in some of the following in a custom infowindow (note that the address variable at the end would probably need to be your actual address or a recordset):
<p><b>Driving Directions</b><br>Enter your start address (street, town, postcode):<form action="http://maps.google.com/maps" method="get" target="_blank">' +
'<input type="text" size=40 maxlength=40 name="saddr" id="saddr" value="" /><br><br>' +
'<input value="Get Directions" type="submit">' +
'<input type="hidden" name="daddr" value="' + address + '"></p>
