Assigned Fulfilment Orders API -- How to update status to "scheduled" ?

[rustcity]

Creating a order fulfilment service app to automate some processes.  

However, on purchase of product to be handled by said fulfilment service, every fulfilment order that is created begins with default status "open".

I ask because I'd like to take advantage of the "fulfill_at" as noted in 

https://shopify.dev/api/admin-rest/2021-10/resources/fulfillmentorder#[post]/admin/api/2021-10/fulfi...

However receive the error 'Fulfillment order must be scheduled.'

How to update the fulfillment_order.status = "scheduled" ?

Cheers

[bashunaimiroy1]

I also have this question.