There is no specific aspect class to add a custom attribute in Metalama, but only the advice factory method IntroduceAttribute. Create an aspect class that calls this advice factory method from BuildAspect(IAspectBuilder<T>).
Namespace: PostSharp.Aspects
Assembly: Metalama.Migration.dll
Syntax
[Obsolete("There is no specific aspect class to add a custom attribute in Metalama, but only the advice factory method 'IntroduceAttribute'. Create an aspect class that calls this advice factory method from 'IAspect<T>.BuildAspect(IAspectBuilder<T>)'.", false)]
public interface ICustomAttributeIntroductionAspect : IAspect