Symptoms
The Additional directives field is missing in Plesk > Domains > example.com > PHP Settings for customer and reseller accounts.
Cause
Only the admin user can set the additional PHP directives.
Resolution
The feature request to allow customers/resellers to manage additional PHP directives:
https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/41973424-new-permission-for-services-reseller-plans-for-the
Please vote for this feature. The top-ranked suggestions are likely to be included in the next versions of Plesk.