Can I put comment and attachment on order timeline or on notes of order using API?

You said you need this link for a user - customer or shop admin?

If for a customer, then do you reference this link in order notification or somewhere else?

Here is an example for order notifications with metafields for a customer:


 Hello, {{ customer.first_name }} 

{% if order.metafields.links.invoice.value != blank %}

 You can download your invoice here: 

 DOWNLOAD 

{% endif %}

Sometimes .value is not required & haven`t tested it, but you get the idea.

If this is for shop admin, then the small box on the right near the link takes you to the url, so it`s still clickable.