I thought CT just sent you a bill for your vehicle tax.

I think you are asking "where do I enter the CT vehicle tax paid in the CT individual return so my client can get the (potential) CT Property Tax Credit?" Is that it?

Try viewing those lines on the form, and right click (jump to input) on one of them.

Welcome to the forum. For future reference, you get lots of room to flesh out your question. It is helpful for us volunteers to know the type of return and state, if applicable, among other things.