C# fluentvalidation | irulebuilder - .when().otherwise()

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C#

I want to create a custom validator for my field UsernameOrEmail, i.e. I need to provide different validations depending on the contents of the field. Is it a username or an email?
How to specify the condition in the code to distinguish?
Can somebody guide me with the syntax of .when() and .otherwise() inside a IRuleBilder method?

What I have tried:

Here is some (pseudo)code:

public static IRuleBuilder UsernameOrEmail(this IRuleBuilder ruleBuilder)
{
   return ruleBuilder
     .NotEmpty()

     .When( ???condition???, () => //in case UsernameOrEmail is an eMail – check if string is not null and string contains the char ‘@’
     // ??? no clue here about the required syntax. Somthing like this: em => em != null && em.Contains(‘@’), () => ???
     {
        .Email() //perform validation for an email address
     }
     ).Otherwise( () => //it is a username
     {
        .Username() //perform validation for a username
     });
}