If you use composer the more recent answer to synchronize with composer is a better option.
PSR-4 namespace roots can specify a prefix which can be configured.
Here is the process:
- Find the root of your namespace e.g
src. - right click Mark Directory as Sources Root.
- Menu -> File -> Settings -> Directories.
- Click the pencil icon or p^ next to your Sources Root.
- Enter the namespace prefix.
See PHPStorm configuring PHP namespaces