This is using simplified xml.
My search string is something along the lines of:
source=mysource.log $myURL$ | ...other phrases... | stats blahblahblah.....
<input type="dropdown" token="myURL">
<label>Select Tag</label>
<!-- Put in your Any value here, if desired -->
<choice value="myURL=* OR NOT myURL=*">Any</choice>
<populatingSearch fieldForValue="mytext" fieldForLabel="mytag">
<!-- I use the eval to create the search text in my populating search -->
<!-- Assuming that you already have a myURL field with the URL information -->
<!-- This also is the entire bit of text for the search my search string -->
<![CDATA[source=mylog.log | eval "myURL=\""+myURL+"\""]]>
</populatingSearch>
When you select one of the URLs--you get:
source=mysource.log myURL="123.123.123.123" | ...other phrases | ....etc
When you select "Any", you get:
source=mysource.log myURL=* OR NOT myURL=* | ...other phrases | ....etc
Cheers!