Refering to a control on a Navigation form

[John Constantine]

How do you refer to a control on a Navigation form in Access 2010 ?

With an ordinary form say it is called “frmPurchaseOrders” and has a
field “PONo”
I could create a button on the “frmPurchaseOrders” form to open a
query for a particular
PONo by setting [Forms]![frmPurchaseOrders ]![PONo] -In the search
criteria of the query

Ok easy enough

But say the form “frmPurchaseOrders” is now part of a navigation form
lets say it is called “FrmNavigate”
What do I put in the query ?
[Forms]![frmPurchaseOrders ]![PONo] no longer works