Create a custom field validation

A question had once been raised in the community regarding customizing a page.

What I need is that on the Dispatch List Revision page I can customize the validation of the "To Transact" field so that it is less than or equal to the "Required" field.

I read the documentation they sent me, but when testing the syntax, it was not possible to produce an error message, or show the user that there was a problem to be fixed.

Here are the syntaxes that my team and I tried to execute:

#{
    if(row.RequiredQuantity != row.TransactionQuantity){
       throw new ValidationException("TESTE")