When adding prospects to an Outreach sequence, you need to provide three specific pieces of information: 1. Prospect ID
2. Sequence ID
3. Mailbox IDThese must be actual IDs from your Outreach account, not dynamic tokens.If you need to find a mailbox ID for a specific email address, you can use the "Lookup Mailbox by Email Address" action first, and then use that ID in your "Add to Sequence" action.
Sources:Add to Sequence with OutreachOutreach integration overview | Documentation | Clay University