From the looks of it QWEB statements does not work in form and tree views.

But this can be done with a computed HTML field on the model and adding it to your views. 

Model:

tracking_number = fields.Char ("Tracking Number") 

tracking_iframe = fields.  Html ("Tracking Preview", sanitize = False, compute = 'get_html') 

def get_html (self): 

  for record in self: 

    record.tracking_iframe = f '<iframe   src = "http://dhl.com/en/express /tracking.html?AWB=} record.tracking_number}" />   

View:

<field name = "tracking_number" /> 

<field name = "tracking_iframe" />

(In the compute function there should be curly brackets around   ecord.tracking_number, forum editor is not accepting it.  )