How do you add and save a POST variable to Qualtrics?

I have some web-based software a user interacts with (made in PHP, HTML, JavaScript). The software records a user's interaction (e.g., web session), and after users complete a task, I would like them to complete a survey. Rather than coding the survey in PHP or Javascript (which can be buggy), I'd like to use Qualtrics. I would like to link their survey output with their interaction data with a userID I define in my software, but it doesn't seem obvious how. I would like to do the following approach:

Submit a form to a qualtrics survey with a userID POST variable.
Save this POST variable as output in Qualtrics.

Is there any way to do this, and if so, how? If there's another way to link the Qualtrics survey data without using POST variables, I'm open to that as well (as long as it isn't GET).

Solution

Qualtrics doesn't accept 'post', so you'll have to use 'get'. Then you can just pass the userID as a field. In the survey flow, define userID as an embedded data variable and don't assign a value to it...it will assign the value from your form. Embedded data variables are saved in your response data.

<form action="https://survey.qualtrics.com/SE/" method="get">
    <input type="hidden" name="SID" value="SV_abc123DEF456" />
    <input type="hidden" name="userID" value="userID_value" />
    <input type="submit" value="Take Survey" /> 
</form>

What is the issue with 'get'? You don't want the user to see their id in the address bar url? If that is the case, you could encrypt it, then call a web service from Qualtrics to un-encrypt it.