Prerequisite: Prior to messaging Klarna on your storefront using the Klarna On-site messaging app, Shopify merchants should first integrate Klarna in checkout. As long as merchants have Klarna Merchant Portal access to accept Klarna's On-site messaging legal terms for their merchant account, if required in their region, this app can be used to display On-site messaging in the Shopify storefront. (This On-site messaging app for Shopify is not compatible with Klarna as a local payment method within Shopify Payments, as that integration doesn't support acceptance of the required Klarna legal terms in the Klarna Merchant Portal.)
If you don’t yet have Klarna API credentials, you can sign up at http://klarna.com/business
Once you have received your Klarna API credentials, instructions to activate Klarna Payments in your Shopify store are available at the Klarna Payments on Shopify section.