Drag it onto the Detail button of the marks card.
This changes my report. I have three tables linked together to create the report. Some of the data changes or turns into an ellipse when I drag this field into the details view.
For you to show the attribute in the URL, it needs to be in the context of the report somewhere. Usually including it on the details shelf will work. If that doesn't work, you might need to add it as a column and hide the column or use another trick.
It's hard to give specific guidance without a Tableau Packaged Workbook and some sample data.