Follow the steps when installing the app from the Shopify App Store. After installation, you'll see a configuration screen where you'll be asked to enter Postcode.eu API keys.
Make sure you have created an account on Postcode.eu. You will receive an API key and an API secret by email. Once you have received these, go to the InStijl Postcode Check app in the Shopify Admin.
In the app settings, enter the API key and API secret from Postcode.eu
Updating the Postcode.eu API keys
Make sure you have the API key and API secret handy. Then in the Shopify Admin, go to the InStijl Postcode Check app.
Go to the app settings, and fill in the API key and API secret for Postcode.eu