Can you be a bit more specific about your situation? Are you talking about a URL field? Are you talking about placing a link within the text of a text field? What’s the purpose of the link – is it to a dynamic page? Is it just a link to a random page?
Depending on what you are doing, there are specific mechanisms available to assign links in a way that will survive conversion of your site to a live, premium site. But there are also limitations.