What you need is to make your URLColumn searchable thus it will be automatically added to the search index. To access it in transformation transformation you just do <%# GetSearchValue("URLColumn") %>
The idea is to have all the columns that you need for rendering of search results in the smart search index. You can access columns that are not in the index in a transformation without putting them into index, but it will be inefficient (essentially the system will run a SQL query to get a value). So basically go to Custom Table/CustomTableName/Search Fields and check your URL field
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