Pin Activity Via REST API (Row Context?)

​@MaheshLakum Thank you for confirming and explaining. 

I made this customization to add an action for this to the Activity form instead:

using PX.Data;
using PX.Objects.CR;
using System;
using System.Collections;
using System.Linq;
using PX.Data.BQL.Fluent;
using PX.Common;

namespace PX.Objects.EP
{
    public class CRActivityMaint_PinViaAPI : PXGraphExtension<CRActivityMaint>
    {

        public SelectFrom<CRActivityPin>.View ActivityPins;


        // API-callable action
        public PXAction<CRPMTimeActivity> TogglePinAPI;

        [PXButton]
        [PXUIField(DisplayName = "Toggle Pin (API)", Visible = true)]
        public virtual IEnumerable togglePinAPI(PXAdapter adapter)
        {
            //PXTrace.WriteInformation("=== TogglePinViaAPI triggered ===");
            var activity = Base.Activities.Current;
            if (activity == null)
            {
                //PXTrace.WriteWarning("No current activity!");
                yield break;
            }

            //PXTrace.WriteInformation($"Activity: NoteID={activity.NoteID}, RefNoteID={activity.RefNoteID}, ClassID={activity.ClassID}");

            PXCache pinCache = Base.Caches<CRActivityPin>();

            // Check current pin state
            PXResultset<CRActivityPin> pins = PXSelect<CRActivityPin,
                Where<CRActivityPin.noteID, Equal<Required<CRActivityPin.noteID>>>
            >.Select(Base, activity.NoteID);

            if (pins != null && pins.Count > 0)
            {
                foreach (CRActivityPin pin in pins){
                  //PXTrace.WriteInformation($"Deleting pin: {pin.NoteID}");
                  pinCache.Delete(pin);
                }
                  Base.Actions.PressSave();
            }
            else
            {
                //PXTrace.WriteInformation("No pin found → Pinning");

                // Pin
                var bAccountPin = new CRActivityPin()
                {
                    NoteID = activity.NoteID,
                };
                pinCache.GetExtension<CRActivityPinExt>(bAccountPin).TargetScreenID = "CR303000";

                pinCache.Insert(bAccountPin);
                Base.Actions.PressSave(); // have to save twice or the two pins will consolidate during audit field addition

                var customerPin = new CRActivityPin()
                {
                    NoteID = activity.NoteID,
                };
                pinCache.GetExtension<CRActivityPinExt>(customerPin).TargetScreenID = "AR303000";

                pinCache.Insert(customerPin);
                Base.Actions.PressSave(); // have to save twice or the two pins will consolidate during audit field addition

                //PXTrace.WriteInformation($"CreatedByScreenID init: {customerPin.TargetScreenID}");             
            }

            // Persist changes
            //Base.Actions.PressSave();
            //PXTrace.WriteInformation("=== TogglePinViaAPI finished ===");
            yield return activity;
        }



        protected virtual void _(Events.RowPersisting<CRActivityPin> e)
        {

          if (e.Row == null || e.Operation.Command() != PXDBOperation.Insert)
                  return;

              var cache = e.Cache;
              var pin = (CRActivityPin)e.Row;
              var ext = cache.GetExtension<CRActivityPinExt>(pin);

                  /*
              PXTrace.WriteInformation(
                  $"[CRActivityPin Persist BEFORE] NoteID={pin.NoteID}, " +
                  $"CreatedByScreenID='{pin.CreatedByScreenID}', " +
                  $"GraphScreenID='{Base?.Accessinfo?.ScreenID}', " +
                  $"TargetScreenID(Ext)='{ext?.TargetScreenID}'"
              );
                  */

              if (!string.IsNullOrEmpty(ext?.TargetScreenID))
              {
                  cache.SetValue<CRActivityPin.createdByScreenID>(pin, ext.TargetScreenID);
                  PXTrace.WriteInformation($"[CRActivityPin Persist AFTER] Forced CreatedByScreenID='{pin.CreatedByScreenID}'");
              }
              else
              {
                  PXTrace.WriteWarning("[CRActivityPin Persist] Missing TargetScreenID; leaving CreatedByScreenID as-is.");
              }

        }

    }
}

This also required a DAC extension:

namespace PX.Objects.CR
{
  public class CRActivityPinExt : PXCacheExtension<PX.Objects.CR.CRActivityPin>
  {
    #region TargetScreenID
    // BQL field declaration – name MUST match the property exactly (case-sensitive)
    public abstract class targetScreenID : PX.Data.BQL.BqlString.Field<targetScreenID> { }

    // Unbound marker used only at runtime (not persisted)
    [PXString(8, IsUnicode = false)]
    [PXUnboundDefault] // optional; keeps the field in cache without DB binding
    [PXUIField(DisplayName = "Target Screen ID", Visible = false, Enabled = false)]
    public virtual string TargetScreenID { get; set; }
    #endregion
  }
}

My use-case is for pinning activities to Business Accounds and Customers rather than Leads.