Macro with message box

[buddythebest]

I want to create a macro (event "onLostFocus" of a field in a form) which views a message box under the condition if this field exists in a query.
Look, I have a query which shows the customers who still did not pay (i.e. pending payment) for an order. I have another form which is the orders form where one enters the customer id and the products he/she is buying. I want to let a message box appear reminding the user if the same customer is making a new order when he already has a pending payment. I thought of creating a macro as an event "On LostFocus" of the customer ID field on the orders form so that if the customer ID exists in the query showing the customers who didnt pay it displays the message reminding the user that he/she didnt pay. I know there should be a condition entered for the macro. Although i tried i couldnt find what condition could i put which is like "[Customer ID] exists in [Pending payments] query".

thank you, please offer your help if you know how to resolve this

 

[KeithG]

You will probably have to create a custom VBA function to check if the customer has a pending payment.

 

[buddythebest]

thanx for bothering, forget it cuz i finally found a solution. I used this as the condition .... and it worked!! (after using other resources for searching of course)
DCount("*","Pending payments","[Customer ID]=Forms![Orders]![Customer ID]")>0