SharePoint: How to add iframe in an application page?

Add <WebPartPages:AllowFraming runat="server" /> in the corresponding master page.

Then you can use iframe in the application page:

<iframe id="iframe1" src="https://navneetchanalia.blogspot.in/"></iframe>

To dynamically set the URL from code behind:

Add following on aspx page:

<asp:Literal ID="ltHTML" runat="server"></asp:Literal>

And on code behind:

protected void SetIframe(string url)

        {

            ltHTML.Text = string.Format("<iframe id=\"testFrame\" src=\"{0}\" style=\"width: 800px; height: 700px;\"></iframe>", url);

        }