Get Field Value from other Nested XML Object

Hi All,

I hope you can help me out.

I have the element Recipient that contains another 2 nested Elements Contact and LetterWindow

Schema: https://mockaroo.com/4ec5ddc0

I am trying to populate line1 through to line3 of Letterwindow using the data from Contact (firstname, lastname)

I have tried concat(field('Recipient.Contact.firstname'),' ',field('Recipient.Contact.lastname')) without success

What do I need to change so this works? Or is this not possible currently?