It would be ideal to be able to map a custom Salesforce field to the matching ID list for primary contact matching. We currently have fields we use as ID fields that aren't recognized by Omatic Cloud so we cannot match or dedupe incoming data based on these fields, making our importing a lot less effective.
Do you know if the custom field in Salesforce is set to be used as an external ID? We recommend that setting in order to display it as a match option. Check out this link in Salesforce and let us know if see any issues.
Product Manager, Omatic Cloud