Users need to give a dev company access to deploy their website.
In other panels like plesk its simply add a user with Create and manage sites, Upload and manage files & Create and manage databases. this will allow them to deploy the site without giving them permissions to view emails or other parts of the subscription out of their scope.
Plesk user roles as example:
Manage users and roles
Create and manage sites
Configure spam filter
Configure antivirus
Create and manage databases
View statistics
Design sites in Presence Builder
Upload and manage files
Create and manage additional FTP accounts
Manage DNS settings
Create and manage mail accounts
