Promotion code can lead some traffic, and increase revenues. At the same time, if a user is ready to buy without a promotion code, you shouldn't distract him with a too much visible promotion code ...
Do you think that in a landing page about a service for pre-ordering lunch should have a registration form or the registration should take place in another page? (after a click on the button "Sign up"...
Checkout processes are usually fairly complicated things, but each one has a very similar set of core steps, such as a header basket summary and some full checkout pages (basket, address and payment ...
For this question, consider the potential negative impact of sending the user off to google for a voucher code only to find expired or duff codes, which would snag and delay them significantly as well ...