if i remember rightly you just advertise and whatever you spend that day comes off the promotional balance,

you can check your spend off the promotion by going to amazon ads, bottom left there is a cog for ‘administration’ then sponsored ads billing then promotion credits,

shows what you have and when they expire

maybe do a £1 campaign for the day and make sure it comes off the balance!

have a good night

Usually they are automatically applied.

Go to your ads console - don’t try on a mobile.
Bottom left corner the cog wheel icon > Sponsored Ads Billing > Promotional Credits