I want to copy the Adress Data in one form (Leads) from one field to another field if the billing address is the shipping address

I want to copy the Adress Data in one form (Leads) from one field to another field if the billing address is the shipping address

Hi,

I try to create an automatic function, that copies the information of the (billing) address fields (street, post code, city, land..)  in the equal fields of the shipping address, if the created checkbox billing address is shipping address ist marked with yes.

Try to do it with workflow, works till the point where I choose the planned action. I chose field actualization, field "Shipping post code" = 
How can I express that it is the Adress Data of the field "Adress post Code"?

Is this a way to get this done? And how?
If not please tell me the right way?

Thank you a lot
Torsten