Want to autofill a field on creation

Want to autofill a field on creation

I use web to lead forms and I'm trying to autofill a field which we dont actually capture on the original form. 

From my lead form we map Lead name.  I want to autofill the Greeting field with the Lead name as well.  Is this possible?  I've created a workflow which is attached.  I was hoping I could just put in a merge tag and it would work.

Your help is much appreciated.