How to configure a foreign key on an owned EF entity's property?

EF Core 3.0

I have the following (simplified) entities in my domain:

class ApplicationUser {
    public int Id { get; set; }
    public string UserName { get; set; }
    // other properties
}

[Owned]
class Stamp {
    public string Username { get; set; }
    public ApplicationUser User { get; set; }
    DateTime DateTime { get; set; }
}

class Activity {
    public Stamp Created { get; set; }
    public Stamp Modified { get; set; }
    // other properties
}

It's not particularly relevant, but it's worth mentioning that ApplicationUser.UserName is a non-primary, unique key. (ApplicationUser actually inherits from ASP.NET IdentityUser.)

I want to enforce that Stamp.Username is a foreign key referencing ApplicationUser.UserName.

If Stamp was a regular, non-owned entity, this would have set up that relationship:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    // Stamp.Username => ApplicationUser.UserName
    modelBuilder.Entity<Stamp>(e => {
        e.HasOne(s => s.User)
        .WithOne()
        .HasForeignKey<Stamp>(s => s.Username)
        .HasPrincipalKey<ApplicationUser>(u => u.UserName);
    });
    ...
}

When I try to create a migration (dotnet ef migrations add) I get an InvalidOperationException saying "The type 'Stamp' cannot be configured as non-owned because an owned entity type with the same name already exists".

How to achieve what I'm trying to do, using Fluent API or otherwise?