Trying to use a lead.token in form follow up URL

Hey guys, 

Wondering if anyone has a solution for hte following:

I have a Marketo form that populates one of the hidden fields (promocode) from the url parameter eg:
www.example.com/?promocode=SamplePromocode 

In the Marketo form, a hidden field (Promocode) is populated from the URL upon submission.

When the form submits, it follows up to a 3rd party application form, which we need to pass the promocode value to the URL, ie:
www.applicationform.com/initiate.aspx?promocode=SamplePromocode


So in the form settings, I've set the followup url using a token:
www.applicationform.com/initiate.aspx?promocode={{lead.Promocode}}

But the redirect url that I get is:
www.applicationform.com/initiate.aspx?pcode=%7B%7Blead.FORM_Promocode%7D%7D&aliId=2882835

Can anyone throw any light on this?
Thanks in advance
Mike