"The Finance Act 2015 amended the Section 231 B of Income Tax Ordinance, 2001 and the revised law is:
231B. Advance tax on private motor vehicles.— (1) Every motor vehicle registering authority of Excise and Taxation Department shall collect advance tax at the time of registration of a motor vehicle, at the rates specified in Division VII of Part IV of the First Schedule."
In form 114(1) of Income tax returns filed voluntarily, put the WHT amount in code 64100301.
In addition to this, you need to fill another form i.e. 165(1) [Statement of WHT filed voluntarily] and put the details in code 64100301.