Phrasee requires a Salesforce Marketing Cloud App / Installed package with the Component type set to API Integration and integration type set to Web App. This app can be created in the specific Business Unit you want to test in.
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To create this app follow the instructions here creating an app named "Phrasee S2S".
​https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-app-development.meta/mc-app-development/install-packages.htm

Once you have created the app add the API Integration Component.
Click on the Add Component button.
Select the API Integration component.
Select the Web App integration type.
Add https://app-ca.phrasee.co/sfmc-callback/ to the URI's
​Check to enable Offline Access
​Click Save.

Add the Marketing Cloud App component to the package.
Click on the Add Component button.
Select the Marketing Cloud App component
Enter a name:
e.g. Phrasee Web App UAT this is what you will see in the Marketing Cloud UI
Enter the following details
Login Endpoint: https://app-ca.phrasee.co/sfmc-login/
Logout Endpoint: https://app-ca.phrasee.co/sfmc-logout/
Click Save
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More info on adding components can be found here
​https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-app-development.meta/mc-app-development/create-a-mc-app.htm

Manage app licencing for use by users in Business Units as per the documentation.
​https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-app-development.meta/mc-app-development/license-installed-packages.htm

Send the Client Id, Client Secret and Authentication Base URI to Phrasee so we can link your Phrasee projects to your Salesforce Business Units.