Function module exit help
I have the following requirement:
Need to check if the material is blocked and return some error code before passing it for the creation of sales order.
The sales order creation is being done using a standard BAPI function module and I need to modify the customer exit (function module exit) to add my logic to check the material.
Can anyone help me in how to proceed with working with function-exits? Where to add my logic?
I am new to this and any help would be appreciated.