How to add classes to code snippet elements?

Questions
1

Hi Guys,
I’ve used the partial view code snippet for a login but would like to add classes to the form and the input elements (Bootstrap site, so for example would like to add .form-control class to the inputs) but can’t find where to do this.
In the macro the razor code is: @Html.TextBoxFor(m => loginModel.Username) and can’t find anything for login model or login controller anywhere.
Thanks for any help!
cheers
Steve

This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/73292-how-to-add-classes-to-code-snippet-elements