How to add City of Detroit form 5121 to my return?

Hi all,

 

I cannot find where/how to complete form 5121 for my wife's Detroit City taxes. Without it, we do show a very tiny refund, but she is only in office a small portion of the year, and we should get back a lot more. 

 

I saw another post from last year, but unfortunately it is not helping me at all here. 

 

If it matters, I am primary on the return, she is the spouse. 

 

To be clear, this is for employer-withheld city taxes.