How to extend DbContext with partial class and partial OnModelCreating method in EntityFramework Core

I'm using EF Core DbFirst approach. My dbContext is created automatically by Scaffold-DbContext command.

I need to add additional DbSets into a dbContext and add into OnModelCreating method of dbContext some additional code but after each scaffolding that added code are erased and I have to add it each time again.

What I want to do is to create another partial dbContext class and mark protected override void OnModelCreating(ModelBuilder modelBuilder) as a partial method

but get errors:

1. A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers.
2. A partial method may not have multiple implementing declarations

Here is a pseudo code:

MyDbContext1.cs - generated by Scaffold-DbContext

public partial class MyDbContext : DbContext
    {
        public MyDbContext()
        {
        }

        public MyDbContext(DbContextOptions<MyDbContext> options)
            : base(options)
        {
        }

        public virtual DbSet<Client> Clients { get; set; }

        protected override partial void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Client>(entity =>
            {
                // ... some code
            }
        }
    }

MyDbContext2.cs - this code I added each time into dbContext after scaffolding:

public partial class MyDbContext
    {
        public virtual DbSet<JustAnotherEntity> AnotherEntity { get; set; }

        protected override partial void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<JustAnotherEntity>(entity =>
            {
                entity.HasKey(e => new {e.Id, e.IdAction, e.IdState})
                    .ForSqlServerIsClustered(false);
            });
        }
    }