// ** CreateOneToManyRequest **
// creates an entity relationship between Account and Campaign entities by using CreateOneToManyRequest
CreateOneToManyRequest createOneToManyRelationshipRequest =
        new CreateOneToManyRequest
        {
            OneToManyRelationship =
            new OneToManyRelationshipMetadata
            {
                ReferencedEntity = "account",
                ReferencingEntity = "campaign",
                SchemaName = "new_account_campaign",
                AssociatedMenuConfiguration = new AssociatedMenuConfiguration
                {
                    Behavior = AssociatedMenuBehavior.UseLabel,
                    Group = AssociatedMenuGroup.Details,
                    Label = new Label("Account", 1033),
                    Order = 10000
                },
                CascadeConfiguration = new CascadeConfiguration
                {
                    Assign = CascadeType.Cascade,
                    Delete = CascadeType.Cascade,
                    Merge = CascadeType.Cascade,
                    Reparent = CascadeType.Cascade,
                    Share = CascadeType.Cascade,
                    Unshare = CascadeType.Cascade
                }
            },
            Lookup = new LookupAttributeMetadata
            {
                SchemaName = "new_parent_accountid",
                DisplayName = new Label("Account Lookup", 1033),
                RequiredLevel = new AttributeRequiredLevelManagedProperty(AttributeRequiredLevel.None),
                Description = new Label("Sample Lookup", 1033)
            }
        };
 
 
CreateOneToManyResponse createOneToManyRelationshipResponse =
    (CreateOneToManyResponse)_service.Execute(createOneToManyRelationshipRequest);