You could do a number of things:
Suppose not all files have all url fields populated, then you might prefer the link to search instead? You get it to do this by using an expression constructed like so:
if(isempty([URL FIELD]),Enter search url here,[URL FIELD])
Now, when you click the link, [URL FIELD] will be used if it is populated, otherwise, the search url will be used.
If you only want the link to show up when a file that has a populated [URL FIELD] is selected, you can select the link in the links manager, click on the "Edit search" button, then click on the "Add new rule" button, paste the following string into the right hand field [=isempty([URL FIELD])]=0 and then simply place [URL FIELD] in the search url field. My experience is that when using this approach, nothing is required in the "Base URL" field.
In these examples, you will need to replace "[URL FIELD]" with your custom field names, keeping the square brackets, of course.
Is that enough to get you started?
-marko
