var rotatortid = 4;

function doSubmit(){
// get the chosen option
selind = document.forms.taxisearch.selectStad.selectedIndex;
// get the value
selval = document.forms.taxisearch.selectStad[selind].value;

// now set this value as action for the form
document.forms.taxisearch.action=selval;

// and submit
document.forms.taxisearch.submit();
}