how to change the value of a field in a dialog ?

dear all,

in the onchange function of a select, i'm trying to change the value of a textinput but can't find the adequate method to do so.

onChange : function( api ) {
    var url = infoTab.get( 'url' );
    url.setValue("abcd");  // something like this
}

any help is well appreciated.

regards,
lionel