In case of a need in providing a dynamic list of email addresses, the pre-job should place into the workspace directory a file with the relevant emails, one email address per line (without a comma between).
The input file should be place into the working directory. In case this field is empty, The file name need to be: ${BUILD_NUMBER}.${BUILD_ID}.emails.input
Please fill this field only in case, you want to hard code the input file name, to be consist across all builds.
Disclaimer: In case you choose to work with the "free vAPI syntax" option, you have the option to supply the list of emails as part of the free vAPI json structure (optionally) - in such cases, this field will be ignored.