AutoMapValidation.cs

static void Main(string[] args)
{
	var config = new MapperConfiguration(cfg =>
	{
		//cfg.CreateMissingTypeMaps = false;
		cfg.CreateMap<DTO_Object, EntityClass>().ForMember(s => s.DynamicFields,
			o => o.MapFrom(s => s.DynamicFields.Select(m => m.Id)));
	});
	config.AssertConfigurationIsValid();
	var mapper = config.CreateMapper();
	
	var source = new DTO_Object
	{
		Id = 1,
		MonthlyPricing = 267.23M,
		DynamicFields = new []{ new DynamicFieldForm{ Id = 1, ServiceId=111 }, new DynamicFieldForm{ Id = 2, ServiceId=222 }}
	};
	try
	{
		var service = mapper.Map<DTO_Object, EntityClass>(source).Dump();
	}
	catch (Exception ex)
	{
		Console.WriteLine(ex.ToString());
	}
}

public class DTO_Object
{
    public int Id { get; set; }  

    [Required]
    public decimal MonthlyPricing { get; set; }

    public IEnumerable<DynamicFieldForm> DynamicFields { get; set; }
}

public class DynamicFieldForm
{

    public int Id { get; set; }

    public int ServiceId { get; set; }
}

public class EntityClass   
{

    [Key]
    public int Id { get; set; }

    public decimal MonthlyPricing { get; set; }

    public virtual IEnumerable<DynamicField> DynamicFields { get; set; }
}

public class DynamicField
{

    public int Id { get; set; }
    [ForeignKey("Service")]
    public int ServiceId { get; set; }
}