How to buy an app without owning a device

My daughter will be getting an iPad for xmas. She already has an apple account.

I want to buy two Affinity apps for her iPad in the Black Friday deal.

How can I do this?

I have just created an apple account so that I can post on this forum. But I don't have any apple devices.

Affinity support have said "contact Apple they can talk you through how to buy the app without owning compatible device"