You should expose a property on your MasterPage which returns the button. In your pages, specify a MasterType directive pointing at your MasterPage. Then in code (like Page_Load) you can do: Me.Master.MyHelpButton.NavigateUrl = "some help url" on each page you would like this for.
If you want to apply this to each page, I would consider creating a base page class that has a property for for the HelpUrl and have each page inherit it (use pageBaseType in web.config to ensure it). Use FindControl to find the Help button and assign it from the property. Then on each page you can do instead:
<%@ Page ... HelpUrl="help1.aspx" %>
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Danny
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