The easiest way is to create a variable for the URL in your MTurk task and fill it with different URLs differing only by the xxx value you mentioned in your previous email.
You MTurk task would include a line like this:
Please click <a href="$URL">this link</a>
And then you provide mturk with a single column csv file that looks like this:
You need to break down your MTurk task into multiple HITs anyway to keep the commission at 20%, so this is a convenient way to address two problems at once. Make sure that participants don't complete different versions of the survey however.