I was able to achieve it using below code:

        public static DelegateFilterBuilder PrefixCaseInsensitive(this IEnumerable<string> value, string prefix)
        {
            return new DelegateFilterBuilder((Func<string, Filter>)(field => (Filter)new PrefixFilter(field, prefix.ToLowerInvariant())))
            {
                FieldNameMethod = (Func<Expression, IClientConventions, string>)((expression, conventions) => conventions.FieldNameConvention.GetFieldNameForLowercase(expression))
            };
        }

Thanks!