// This javascript sets the default value of a lookup field identified 
// by <<FIELD DISPLAY NAME>> to the value stored in the querysting variable
// identified by <<QUERYSTRING VARIABLE NAME>>
// Customize this javascript by replacing <<FIELD DISPLAY NAME>> and 
// <<QUERYSTRING VARIABLE NAME>> with appropriate values.
// Then just paste it into NewForm.aspx inside PlaceHolderMain
 
_spBodyOnLoadFunctionNames.push("fillDefaultValues");
 
function fillDefaultValues() {
  var qs = location.search.substring(1, location.search.length);
  var args = qs.split("&");
  var vals = new Object();
  for (var i=0; i < args.length; i++) {
    var nameVal = args[i].split("=");
    var temp = unescape(nameVal[1]).split('+');
    nameVal[1] = temp.join(' ');
    vals[nameVal[0]] = nameVal[1];
  }  
  setLookupFromFieldName("<<FIELD DISPLAY NAME>>", vals["<<QUERYSTRING VARIABLE NAME>>"]);
}
 
function setLookupFromFieldName(fieldName, value) {
  if (value == undefined) return;
  var theSelect = getTagFromIdentifierAndTitle("select","Lookup",fieldName);
 
// if theSelect is null, it means that the target list has more than
// 20 items, and the Lookup is being rendered with an input element
 
  if (theSelect == null) { 
    var theInput = getTagFromIdentifierAndTitle("input","",fieldName);
    ShowDropdown(theInput.id); //this function is provided by SharePoint 
    var opt=document.getElementById(theInput.opt);
    setSelectedOption(opt, value);
    OptLoseFocus(opt); //this function is provided by SharePoint 
  } else {
    setSelectedOption(theSelect, value);
  }
}
 
function setSelectedOption(select, value) {
  var opts = select.options;
  var l = opts.length;
  if (select == null) return;
  for (var i=0; i < l; i++) {
    if (opts[i].value == value) {
      select.selectedIndex = i;
      return true;
    }
  }
  return false;
}
 
function getTagFromIdentifierAndTitle(tagName, identifier, title) {
  var len = identifier.length;
  var tags = document.getElementsByTagName(tagName);
  for (var i=0; i < tags.length; i++) {
    var tempString = tags[i].id;
    if (tags[i].title == title && (identifier == "" || tempString.indexOf(identifier) == tempString.length - len)) {
      return tags[i];
    }
  }
  return null;
}