I know I can add a custom field and put in a url or location to a business case after it has been produced, but I would like a way to prompt the idea submitter to be prompted to download and link to a business case template (and then upload or link it to their idea)
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As you noted, there are a couple workarounds with custom fields. As described, one would be to include a custom field for you to enter information after an idea has been submitted.
It's also possible to add a required custom field for a user to enter a link to the business case template. Note that for ideas submitted through the portal, this field can be made required (but ideas submitted through the application will not require this field). Here's a blog post with more details: http://blog.aha.io/index.php/just-launched-improved-idea-portals-now-with-required-fields/
Because of the noted workarounds, we will mark this idea as Already exists.