You don't say if this is a Continuous form. If it is, you can just refer to
the Invoice Number field directly, as the form knows which is the current
record. Something like:

DoCmd.OpenForm "form name",,,"[Invoice Number]=" & Me.txtInvoiceNumber

Of course, I don't know the name of your form, fields and controls, so
you'll need to put in your own names there. This assumes the form you're
opening is bound to the invoice data table; what you're doing is adding a
WHERE clause to the form's RecordSource when it opens, displaying only the
Invoice Number you're interested in.

If the form isn't bound to the invoice table, you can instead pass the
invoice number in the OpenArgs parameter of OpenForm and set the form's
RecordSource in the Form_Load event, using the passed invoice number.

HTH,

Carl Rapson