fulfillment_request.json API comes back with "...assigned fulfillment service must be of api type"

[agon04]

Trying to migrate my App from from "/orders/{order_id}/fulfillments.json" to "/fulfillment_orders/{fulfillment_id}/fulfillment_request.json".
My Order Line Item looks like this:
{
id: 999999,
admin_graphql_api_id: "gid://shopify/LineItem/999999",
fulfillable_quantity: 2,
fulfillment_service: "manual",
fulfillment_status: null,
gift_card: false,
.......
}

My API request is "POST .../fulfillment_orders/777777/fulfillment_request.json" with the payload below:
{"fulfillment_service":{"message":"Express","fulfillment_order_line_items":[{"quantity":1,"id":999999}]}}

Getting the following reply:
{"errors":["The fulfillment order's assigned fulfillment service must be of api type"]}

Questions:
How do I change Order Line's fulfillment_service attribute from "manual" to "api" or whatever is required?
Going forward I'd like this attribute to be "api" (or alike) - is this Variant-based setting?