In order to connect to Salesforce Marketing Cloud /ExactTarget to Grow, you will need a "Client ID" and a "Client Secret." To acquire these credentials, you will need to login to the Salesforce Marketing Cloud App Center.
Salesforce has an article that describes the process. Here, you can find a summary below.
Once you have logged in to the App Center, click the blue create a new app button at the top right of the screen, then choose API Integration.
After selecting API Integration, the first step will have you enter in a name, description, and package. These can be whatever you would like. When you're done, click next.
In step 2, choose your Salesforce Marketing Cloud account to integrate with and click next again. When you link the account, an authorization box will pop up. Go ahead and log in to your account and connect them.
For step 3, data access, you will need to give "Read" permissions to lists and subscribers and audiences (both located under contacts), and data extensions and file locations (both located under data). Then click Next.
This will bring you to a summary page where you can see all of the settings you've connected. Double check it, then click Finish.
You should now be able to see your client ID and client secret under the authentication and integration section of the overview page. You will also want to take note of the Authentication Target, right next to the client ID and client secret. This will come in handy when you connect to Grow.
Copy and paste your client ID and client secret into Grow. Under Instance, select the instance from your authentication target. Hit submit, and you are ready to go!
If you have any questions, email us at firstname.lastname@example.org or chat in and let us know.