Src assignment problem of iframe (server side)_HTML/Xhtml_Web page production

I encountered this problem today. The iframe on the server side reassigned the src value, added an ID to the iframe, and added runat=server

1. Assign the src value of the iframe through JS

2. If It is necessary to change the src of the iframe when the page is loaded in the C# background. You can apply for it as a server-side control through

such as