Hi Johnny,
You had me confused a little with connecting an SF field to a Marketo token. The field actually gets setup in Marketo, so the field maps to the field and of course any lead / company field in Marketo can be used as a token.
That being said, there's a few things to know:
If you want a field in Marketo that's synced with SF, you should NEVER create it in Marketo. The trick is to create it in SFDC and then sync it to Marketo. If you create a field manually in Marketo it will never sync with SFDC.
If you are sure the field has been set up to sync (which by the sound of it is the case, as Marketo tells you the field already exists) you need to be aware that there are several tabs it may have ended up in. Make sure to check the SFDC Lead Fields and SFDC Custom Fields for it.
Still no luck? If the field exists inside a custom object, it will not show as a separate field. Check with your SFDC admin whether this is the case and if so, in which custom object it is set up. These objects are treated quite differently in Marketo. You access them in a smart list as "Has <custom object name>" and then with constraints you can add criteria for your specific field within the object. If this is the set up you have, it is important to know that you cannot use these fields directly as a token.
I hope this helps in your search.
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