We've opened up the ability for any published Shopify marketing app to be embedded directly into the Shopify admin through the marketing activities API.
You can build your own app extension today to be on the list of marketing integrations.
You can find more information about creating an app extension using the marketing activities API in the Shopify Help Center
At a high level, the implementation requires the following steps.
If you have any questions about this update, then contact support through your partner dashboard or request support in our Shopify APIs and SDKs discussion board.