I'm showing a message box if customer's SQL is version 2005.
We want to redirect them to a help web site we have, but I don't know how to insert and http link in the message box.
Indeed, the message box displayed by our predefined custom action does not support links on it. Do you want the user to be redirected to a web page when he clicks a button of the message box or when he clicks the link displayed by the message box?
In order to achieve this you can try to implement the last five steps described in the How do I make sure an edit box is not empty article and place your link on that dialog, also use your SQL property as a condition.