Thay doesnt seem tot change the default operator selected in the ui though

YourMethodName (string propertyName, string keyword)
        {
            var descriptorcount = radGridProductInfoListView.FilterDescriptors.Count;
            if (descriptorcount > 0)
            {
                for (int i = 0; i < descriptorcount; i++)
                {
                    radGridProductInfoListView.FilterDescriptors.RemoveAt(i);
                }
            }

        FilterProductInfo = new TextFilterDescriptor();
		
        FilterProductInfo.IsCaseSensitive = false;
        FilterProductInfo.Operator = TextOperator.Contains;
        FilterProductInfo.PropertyName = propertyName;
        FilterProductInfo.Value = keyword.Trim();
		
        radGridProductInfoListView.FilterDescriptors.Add(FilterProductInfo);
        }

That's how I did mine. Needs to create new instance of TextFilterDescriptor because the grid will treat the new filter as AND.