Take a look at this Github repo. I have some examples with references to help documentation on how to customize the form output and it will customize EVERY form you output, not just one.
In the /Components folder there are two .cs files for form builder customizations and markup injection. This files will allow you to set overall form options and override individual form elements, fields, etc. This is a v13 .net core example but you can customize it for v12 MVC5 as well.