How to set up payment processing app with GraphQL mutation?

[OlegarioF]

Hello I am migrating to the Shopify apps and there are some things that I cannot understand one of them is:

• The documentation says I need to make a POST request in order to set my app as ready to process payments

But I don't know what value to use for the `externalHandle` param and how I should send it (as a query param, as part of the body, etc).

Can someone help me with this?

Thanks in advance