When you first sync SFDC to Marketo, you have the option to map your fields. After that, you will need to create fields in SFDC first and make them available to the Marketo Sync User. That field will then be automatically synced to Marketo. More info here:
Thank you both. I did review the documentation, but that field is just not there. I am thinking that maybe got created in Marketo first (?), and hence messing the mapping and all. I will ask the Marketo team for support as I did not know they could help with that.