You can create a rule to include a specific IOSS number against an order automatically via rules.

Example Use Case - Submit eBay IOSS number for only shipments which have been imported from eBay.

This rule can be replicated for different marketplaces such as Amazon,Etsy etc. 

1. Set the First condition

1. Go to Settings > Rule > + Add a new Rule
2. Use the dropdown menu to set the condition "Destination Country Code" In "European Union Countries"

2. Set the Second condition.

1. Click on +Add Another Condition to this rule. 
2. Use the dropdown menu to set the second condition e.g. AND if the Order Value is Less Than 150.00

3. Set the Third condition.

1. Click on +Add Another Condition to this rule. 
2. Use the dropdown menu to set the third condition e.g. And if Integration equals eBay

3. Set the action 

1. Select the Action "Set Shipper Tax Number" using the drop-down menu
2. Set Value to "IOSS - Import One-Stop-Shop" 
3. Add in the IOSS number to be used 

This will now be automatically supplied to DHL when an order meets the above condition. You can replicate this rule and change the "Integration" to a different one to accomodate any other marketplaces.