We have a custom button that came with the installation of a managed package. We are using this custom button on a page layout to open a new page with a PDF. We want to display an error message when the user clicks on this button and one of the custom fields is blank, prompting users to add a value to the field first.
We can't make the field required as there are other business functionalities that depend on it, so the field must remain non-required.
Any help would be appreciated. Thanks.