How do you do that?
I don't know why Citi still has this service, and seems to keep developing it (it used to be a Flash page, and since Flash is out commissioned, they rewrote the whole service into HTML).
So if you are in US, get a Citi CC or something like privacy.com.
You can set expiration time and daily spending limit for the numbers. Also optionally assign a name for each number.
Note that I have two Citi CC but only one card has this service. So some card may not. The one available to me is the double cash card.