Form Class declaration using Extension

Good morning,

As i am trying to add number sequence to customer group.

I got stuck in one place.

in ax 2012 we used to mention .

NumberSeqFormHandler   numberseqformhandler;  is used to declare in form --> Class declaration.

In D365 i created extension form for custGroup.

Now where should i write Numberseqformhandler as global.

Should i create new class for CustGroupForm extension and should i writein that class ?

Please provide some light on this.

Regards.

Have a great day.