AnswerKey on string contains

I have the following question that I have on Mechanical Turk question:

<?xml version="1.0"?>
<QuestionForm xmlns="http://ift.tt/1oLVbcb">
    <Overview>
        <Title>Provide the correct IMDb url</Title>
    </Overview>
    <Question>
        <QuestionIdentifier>Q1</QuestionIdentifier>
        <IsRequired>true</IsRequired>
        <QuestionContent>
            <Title>Provider the correct IMDb URL</Title>
        </QuestionContent>
        <AnswerSpecification>
            <FreeTextAnswer>
                <Constraints/>
                <NumberOfLinesSuggestion>1</NumberOfLinesSuggestion>
            </FreeTextAnswer>
        </AnswerSpecification>
    </Question>
</QuestionForm>

I want to accept answers that have the string "http://ift.tt/1CLaOn9" contained in it. How would I create an AnswerKey to do this. Conceptually:

<AnswerKey xmlns="http://ift.tt/1Ebv0Tm">
  <Question>
    <QuestionIdentifier>470015399</QuestionIdentifier>
    <AnswerOption>
      <SelectionIdentifier>**CONTAINS "http://ift.tt/1CLaOn9"**</SelectionIdentifier>
      <AnswerScore>1</AnswerScore>
    </AnswerOption>
  </Question>
</AnswerKey>

How would I do this?