create unique id(custom id ) in sql

try to use this,  EF Core (Code-first) should work. if you want a third property as autogenerated

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity()
    .Property(p => p.YourCustomProperty)
    .HasComputedColumnSql("'FM' +[UserId]");
}

If you want to make userId as computed column then 

[Key]
[DatabaseGenerated(DatabaseGeneratedOption.None)]
public string UserId { get; set; }            

[Required(ErrorMessage ="Please Enter Username..")]  

 [Display(Name = "UserName")]  

public string Username { get; set; }  

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity()
    .Property(p => p.YourCustomProperty)
    .HasComputedColumnSql("'FM' +Scope_Identity()");
}