checkbox using Html string builder

I am working on c# asp.net web App. I have created checkboxes using html string builder.

how can I assign controller to the input element, and then how can I access the checkbox from my c# code. Or do you have another way of doing it?

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