You need to update a couple of files to define the payment method facade in the payment plugin.

There is a naming convention between Adyen and Magento payment methods. A Magento payment method for Adyen plugin should start with adyen_ and followed by the payment method type name scalapay_3x.

• etc/di.xml: Virtual types for the payment method model needs to added here
• etc/frontend/di.xml: Payment method type name needs to be added to this list (adyen_scalapay_3x => scalapay_3x)
• etc/config.xml: Payment method default configuration block needs to be defined here. You can check this document to find the correct configuration values
• etc/events.xml: Event for payment data assign observer can be defined here.
• etc/graphql/di.xml: GraphQL data provider needs be added for adyen_scalapay_3x as Adyen\Payment\Model\Cart\Payment\AdditionalDataProvider\AdyenPm
• etc/payment.xml: Let multipshipping use this payment method
• view/frontend/layout/checkout_index_index.xml: Extend the block to allow billing address selection for adyen_scalapay_3x.
• view/frontend/multishipping_checkout_billing.xml: Define form template for multipshipping payments
• Download the payment method logo from Adyen Checkout CDN and place it under view/base/web/images/logos directory as scalapay_3x.svg

After implementing those changes run the following commands.

bin/magento setup:di:compile
bin/magento setup:static-content:deploy -f
bin/magento adyen:enablepaymentmethods:run
bin/magento cache:clean

You can also check pull-request #2918 on the open-source Adyen Magento 2 plugin repository and find generic implementation document here if required.