When sending internationally you may notice that IOSS numbers are required to allow some carriers to accept and ship an order, as such, many platforms have put in place fields for this information to be filled out in.
However, Etsy have currently confirmed that it's not possible in their system for the IOSS number to be passed through to Shiptheory on a per order basis. That doesn't mean that Shiptheory can't send this, as there are a few alternatives in Shiptheory that can still ensure the information is sent to you carriers;
The IOSS number can also be filled out in your company settings and will then automatically apply and send out to carriers when needed, this page can be found here.
However, you can also use the Shiptheory rules to update the IOSS on a more per order basis, this can be done in the "Update Shipment Details" option: 
For more information on that, please see our guide: Using Shipping Rules to set IOSS and other tax numbers
If you have any questions on the Shiptheory side, please feel free to Contact Support.