Unfortunately, there is a lack of documentation for this specific setup.  We had to have a consultant work with us on a couple before we were knowledgeable enough to do our own.  I will try to put some basic instructions together for you.  I am working in IFS CLOUD so may look different.

Also, this may help.  

Many thanks for your support

Here is a very basic process.

You need to create a new custom event.

1. Assign an ID
2. Make sure it is enabled.
3. Select the entity and table that the data resides in that if changed the event will trigger.
4. Assign when you want it to fire.  In this case, I want it to fire when the combination of the three objects selected below are created.  This is during receiving.

You need a New Event Action to to run when the criteria from the custom event is met.

1. Select type of event action.  It this case, it is a sql statement.
2. Select the custom event you just created.
3. Make sure enabled.
4. The conditions should populate from the customer event.
5. Since the event type is a sql statement, you include your sql statement here.
   1. begin
      if &NEW:SOURCE_QTY_ARRIVED<>&NEW:INV_QTY_ARRIVED AND &NEW:INV_QTY_ARRIVED <> 0 then
      Error_SYS.Appl_general('PurchaseOrderLine','Inventory quantity of &NEW:INV_QTY_ARRIVED and &NEW:SOURCE_QTY_ARRIVED do not match.  Return to receiving.');
      else Client_SYS.Add_Warning('PurchaseOrderLine','Verify quantity of &NEW:INV_PART_NO &NEW:SOURCE_QTY_ARRIVED');
      end if;
      end;
   2. This statement essentially checks first to make sure the Source Qty and Inventory Quantity match...we have seen issues with this.  If it does not match, it will not let them proceed until fixed.  If it does match, it uses the Clietn_SYS.Add_Warning command. 

Play around with this a little.  The main thing is finding the commands.  Client_SYS.Add_Warning was what I need the help from our consultant on.  It allows OK or Cancel.