Not sure if your client is married but if they are, on the Info worksheet - Part XI - you should check the box that says 'Separately' to break out the income. You also will need to allocate it on the City Wg/SE Alloc worksheet. 

Your client should have received a completely separate, NYC 1127 Wage statement which you will need as well.

Lastly, on the NYC-1127 form smart worksheet, you need to check one (or both of these boxes) Married Filing Jointly for state purposes & Separate for 1127; Employee was not a full year employee of NYC, then enter the amount of income attributable to NYC for the year. 

To your point  you will also need the IT-360.1. 

Hope this helps.