In a HTTP listener, how do I respond to an AJAX request?

When an AJAX request is made to Mirth, I want to be able to respond with a message.

For example, my request was made by XMLHttpRequest.

Code:

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var xhr = new XMLHttpRequest();

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How can I respond in Mirth so that

Code:

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xhr.responseText

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will contain a message?

In a HTTP listener, how do I respond to an AJAX request?