Good morning, 

I would like to create a custom event on my environment. The event should work like this:

- User can pre-invoice when price = 0
- When the invoice is preliminary and the price still = 0 should pop up a message or an error about the execution of the operation with the information that the price is 0

I would also specifically like to limit the operation of the event to a specific series of invoices, because, for example, invoice adjustments already the user should be able to create with a price of 0 and this determines the series. 

My question is that I need any guidance to create this event:

- On what entity and table to post the event? Do I include any specific conditions?
- Or does anyone have such a solution working at their place and would like to help me implement it?