REST API doesn't update SFDC custom variables

I have a smart list which sends out reminder emails when the sfdc field "Downloaded Product?" changes to Yes. When I do this manually by checking the checkbox on the sfdc website, all is well and good. The issue is when I try to update this information through the marketo rest api.

It seems that the sfdc variables aren't being updated. I'm not sure if the value that a salesforce checkbox is expecting is a "Yes"/"No" string or a true/false boolean, but neither one is changing the value in sfdc.

Here's an example request:

POST to MARKETO_ENDPOINT/leadCapture/save
{
                "Email": XXXXXXX@XXXXX.XXX,
                "Downloaded_Product__c": "Yes",
                "formid": XXX,
                "lpId": XXXX,
                "subId": XXX,
                "munchkinId": XXX-XXX-XXX,
                "_mkt_trk": "return"
}

Is there any reference on how to update sfdc variables that i missed?