On Android:
A transaction number looks like this: GPA.1234-1234-1234-12345. You can find it in the receipt you received from Google via email after completing your purchase in the game. You can also go to https://pay.google.com and tap on an order to see its details.
On iOS:
Your order ID can be found in your Purchase history. There is more information on this on the Apple support website: https://support.apple.com/en-gb/HT204088
Via Huawei AppGallery:
Open AppGallery, then tap on your profile at the bottom of the screen and select "Purchase history"