Article from Netsuite Help:
https://333032.app.netsuite.com/app/help/helpcenter.nl?topic=EDIT_CUSTCENTER
And few screenshots from me:
To add new menu item to custom center/tab:
Navigate to Customization >> Centers and Tabs >> Center Categories >>
Find the Category / Center you want to add link to. Click Edit:
3. Add/Edit the link
You would need here a name of the suitelet deployment.
Job done:
BR,
Alex