You want to create an external interface routing rule.

Fulfillment line and E. Task layer service

Fulfillment line and task layer service are two attributes that are available for selecting a fulfillment system connector. A fulfillment line is a line on a sales order that represents a request for a product or service.A task layer service is a predefined web service that performs a specific task in the fulfillment process, such as shipment, invoice, or payment1.You can use these attributes to create external interface routing rules that route fulfillment requests to your fulfillment system based on the sales order, fulfillment line, or orchestration process attribute2.

References:

Fulfillment Line

Task Layer Service

Route Requests from Order Management to Fulfillment Systems