square.onMerchantCustomAttributeDefinitionOwnedCreated subscribes to merchant.custom_attribute_definition.owned.created for the selected seller. It requires MERCHANT_PROFILE_READ.
Example
automations/square-on-merchant-custom-attribute-definition-owned-created.automation.ts
Event
A custom attribute definition was created by your application. The application that created the custom attribute definition is the owner of the definition and corresponding custom attributes. The signal resolves to the officialSquare.MerchantCustomAttributeDefinitionOwnedCreatedEvent payload with type narrowed to merchant.custom_attribute_definition.owned.created.