The above type handler can be installed into SqlMapper using the AddTypeHandler method.
SqlMapper.AddTypeHandler<IHtmlString>(new IHtmlStringTypeHandler());
Type inference allows you to omit the generic type parameter:
SqlMapper.AddTypeHandler(new IHtmlStringTypeHandler());
There's also a two-argument overload which takes an explicit Type argument:
SqlMapper.AddTypeHandler(typeof(IHtmlString), new IHtmlStringTypeHandler());