Hi-
It looks like InfoPath is reading this as a literal string:
ChangeSubmitUrl /submit=SharePoint Library Submit /url=http://<site location>/my:dept
And not as a formula.
To have InfoPath use this as a formula, you'll need to click fx when setting the value of the field to get the Formula screen. Then you can use the concat function to create this
concat("ChangeSubmitUrl /submit=SharePoint Library Submit /url=http://<site location>/",my:dept)
my:dept is underlined and in bold to show that you would use the Insert Field or Group option to add that field.
This way InfoPath interprets this as a formula, and gets the value of the my:dept node.
Hope this helps!