For this, you will need to have the asset ID(s) and the order ID. You can find assets via this API.

Attach

Use the order ID and send a POST request with the asset ID to:

https://app.digital-downloads.com/api/v1/orders/:id/attach

https://app.digital-downloads.com/api/v1/orders/82664d96-6dfd-4343-aaaa-05c46f412a5b/attach

JSON

{
    "assets": [
        "82664d96-6dfd-4343-96b0-05c46f412a5b", 
        "82664d96-6dfd-4343-96b0-05c46f412a5x"
    ]
}

You will receive a 201 response code.

Removing attached assets

Use the order ID you wish to remove and send a POST request with the asset ID to:

https://app.digital-downloads.com/api/v1/orders/:id/detach

https://app.digital-downloads.com/api/v1/orders/82664d96-6dfd-4343-aaaa-05c46f412a5b/detach

JSON

{
    "assets": [
        "82664d96-6dfd-4343-96b0-05c46f412a5x"
    ]
}

You will receive a 201 response code.