EF Core Fluent API to map collection of Users

I have a Notification class

public class Notification
{
    public int NotificationId { get; set; }

    public string NotificationMessage { get; set; }
    public DateTime NotificationSentOn { get; set; }

    //TODO: not sure how to map this in fluent api
    // a Notification can go to many users
    public ICollection<ApplicationUser> ReceivingUsers { get; set; }
}

and the extension of ApplicationUser

public class ApplicationUser : IdentityUser
{
    public ApplicationUser()
    {
        IsAuthor = false;
    }
    public string Firstname { get; set; }
    public string Lastname { get; set; }
    public Gender Gender { get; set; }
    public DateTime? DateOfBirth { get; set; }
    public DateTime RegisteredDate { get; set; }
    public bool IsAuthor { get; set; }

    // foreign key to UserProfile using the string ID from ApplicationUser
    public UserProfile MemberProfile { get; set; }
    // collection of notifications for this user
    public ICollection<Notification> Notifications { get; set; }
}

Here is the error relating to the Notifications property in the ApplicationUser class

Unable to determine the relationship represented by navigation property 'ApplicationUser.Notifications' of type 'ICollection'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

I believe the relationship should be one-many , i.e. one Notification goes to many ApplicationUsers, but my normal pattern in Entity Configuration doesn't work, I must be missing something in one of the classes.

I am not sure how to map both the Notifications collection or the foreignkey relation to UserProfile using the fluent API ( I am using EntityConfiguration classes using IEntityTypeConfiguration interface)

Update Per Camilo's answer, I updated my Entity Configurations to include the NavigationUser table setting the primary key as follows

public class NotificationUserEntityConfiguration : IEntityTypeConfiguration<NotificationUser>
{
    public void Configure(EntityTypeBuilder<NotificationUser> builder)
    {
        builder.HasKey(u => new { u.ApplicationUserId, u.NotificationId })
            .HasName("PK_NotificationUser");

        builder.Property(u => u.NotificationId)
            .ValueGeneratedNever()
            .IsRequired();            

        builder.Property(u => u.ApplicationUserId)
            .ValueGeneratedNever()
            .IsRequired();

    }
}

This returned the following from the db creation script

It created a ForeignKey in the ApplicationUser table

table.ForeignKey(                        name: "FK_AspNetUsers_Notifications_NotificationId",
        column: x => x.NotificationId,
        principalSchema: "MachineryCtx",
        principalTable: "Notifications",
        principalColumn: "NotificationId",
        onDelete: ReferentialAction.Restrict);

and a ForeignKey in the NotificationUsers table back to notifications

table.ForeignKey(                       name: "FK_NotificationUser_Notifications_NotificationId",
      column: x => x.NotificationId,
      principalSchema: "MachineryCtx",
      principalTable: "Notifications",
      principalColumn: "NotificationId",
      onDelete: ReferentialAction.Cascade);