Find the "Sign-in redirect url"

Most of the identity providers need a Redirect URL and sometimes you also need a Manifest URL. You can find this url at your defined SSO entry in Shopware administration at Settings > Extensions > Storefront SSO by selecting on of the SSO entries. Additionally you can find a Login URL at the URL box. The Login URL can be used to implement a Login button outside of Shopware to login at an identity provider and to redirect to the shop.

Every sales channel and every language has it's own urls. Copy the urls from Shopware admin and enter it in identity provider configuration on site of the provider.