Citi does not give instant card number.

You will need to ACTIVATEA the new card before you can add it to an existing citi online account. Dont know what "new citi account" you have created - I suppose it is one of the screens ask if you want to get online access... Should not do that because it often creates complication when you actually receive the card, activated it, then you have trouble to "add" it to the "new account" you created.

Wait till you receive the physical card, call to activate it (it is automatic system, but sometimes if you activate it online, system would flag for security block without you even know it - happened on me when I returned to Citi after a long period. Call from the telephone number used on your application to activate the card seems to avoid this potential mess.
Once the card is activated, see if you can link it to the "new account" you have created. If you encounter issue, abandon that "new account" and start the registration anew - but unfortunately you will not be able to use the same userid that has used to create the "new account".

It is the lousy Citi IT that has all sorts of quirks that you just need to learn and deal with them.