Custom Validation Attribute for model C#

I have a model:

public class SendEmail{    public bool IsScheduled { get; set; }    public DateTime ScheduleDate { get; set; }    public List<ScheduleAttachement> ScheduleAttachement {get; set;}}public class ScheduleAttachement{    public string Name { get; set; }    public string Attachement { get; set; }}

I want to do a custom validation to check if bool IsScheduled == true, ScheduleAttachement must contain a value. If not throw 400 bad request