How do I connect to a custom domain?

When you complete the connection between Mailchimp and Salesforce, you are typically redirected back to Salesforce automatically without any issues.

However, if you have access to multiple Salesforce orgs or use a custom domain, you may instead be redirected to the Salesforce login page. In that case, you’ll have a brief window to manually enter your username and password. Even if prompted to log in again, your original Salesforce session remains active and can still be used.

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Or follow these steps: 
1. Log into your org (sandbox or production)
2. Copy the URL of just the domain (yourcustomdomain.my.salesforce.com or similar)
3. Connect to Mailchimp as per the instructions
4. When you get to the Salesforce login step, paste that domain from step 2 in place of the test.salesforce.com or login.salesforce.com in the URL. Keep everything after .com, including the slash

It will use your current logged-in session instead of asking you to authenticate a new session.