Copy custom field from leads

Hello

I’m trying to create my very first workflow. I have a custom field in the leads module called party-date (which is a date-type field). After having converted the lead to an opportunity I want the workflow to create an event on the party-date when I change the opportunity-stage to booked.

I’ve found the matching condition and action. But I can’t select my custom date field as start-date for the event. Do I have to link it somewhere?

Regards