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If you need to update a URL field in a links list (or any other list) from SharePoint Designer, you need to know this little trick.  Create a string that contains the URL and description like this:

URL, Description

and then update the field with this string.  Notice there is a comma AND A SPACE after the URL.  There are no brackets, curly braces or octothorpes (# signs) involved in the string.

Using the string builder, you can see the "input" and the "output" of the workflow.