Workflow Rules - Field Update

Hello!

I am trying to get Workflow Rule, triggered when donors make a donation on our webpage, to do Field Update in the Contact field, where Contact = Organization Name.

Our Soapbox-Salesforce integration creates a new Opportunity and populates the Organization Name in the (new) Opportunity with the donor's name filled in on our web donation page. However, in this new Opportunity that is created, the Contact field is left blank. It should be filled in with the Firstname Lastname from the donation page.

Hence, I have created a Workflow Rule, and am trying to add Workflow Action so that Contact = Organization Name. However, I do not see the option of referring to Contact in the New Field Update's Field to Update.

The Field to Update box shows Opportunity object, but in it I do not see Contact field.

Any ideas/suggestions?

Thanks in advance!

Ken.