how to access check box of a place holder

I am working on a web app using ASP.net C#. I have created check boxes using HTML string builder in my page load.

StringBuilder html = new StringBuilder();
 html.Append(" <input type='checkbox' class='custom-control-input' runat='server' id='check1"'>");

In onClick function, how can I access the id of my checkbox which I have placed in html placeholder?