If you create a custom object, you will need to use the custom Salesforce ID that is created for that custom object when using List Builder. Follow these steps to find the ID:
  1. Log into Salesforce
  2. Navigate to the specific custom object in question
  3. Click on the custom object
  4. Highlight and copy the 15-character Salesforce ID from the custom object's web URL
  5. Convert the 15-character Salesforce ID to 18 characters using the following tool: https://www.adminbooster.com/tool/15to18
  6. Use the 18 character Salesforce ID to perform the List Builder search
If you have any questions, contact Support.