Blockchain.info testnet payment APIs

I have created a testnet wallet on testnet.blockchain.info and added some testnet bitcoins to it.

I want to test the payment APIs so I am following the documentation on how to Generate a receive address. Yet I do not see a way or an URI to register a testnet receive address.

I tried to use it as is and I've put my testnet xpub in the query but I get this error:

{
    "message": "Invalid xpub format",
    "description": "Ensure you provide an xpub key, rather than an address"
}

Anyone had success using blockchain's payment APIs with testnet before?

Blockchain.info doesn't support API for Testnet network.

That API Pages for Mainnet network. What you can use blockcypher as an alternative, they're supporting Testnet for bitcoin.