Set The Value Of A Report Field From Code

I am using this code on a button, to print a report:

Private Sub buttonPrint_Click()
Dim P1 As Integer
Dim R1 As Integer
Dim stDocName as String

R1 = MsgBox("Print;", vbYesNo)
If R1 = vbYes Then
If stDocName = "FIRST REPORT" Then

P1 = MsgBox("Printing options;" & vbCrLf & vbCrLf & _
" YES: Option 1" & vbCrLf & vbCrLf & _
" ΝΟ: Option 2", vbYesNo "Printing Options")

If P1 = vbYes Then
DoCmd.OpenReport stDocName, acNormal
End If
Exit Sub

Here is the difficulty I need help to overcome:
In report "FIRST REPORT" there is one field called txtSet1, Boolean.
Can I set the value of this field to either True or False from this point in my code ?

If P1 = vbYes Then
set Reports![FIRST REPORT]!txtSet1.value = True
DoCmd.OpenReport stDocName, acNormal
End If
Exit Sub
I know that the answer is NO because the above does not work. I think the problem is that because the "FIRST REPORT" is not opened until line
DoCmd.OpenReport stDocName, acNormal
is executed, the line
set Reports![FIRST REPORT]!txtSet1.value = True
can not find "FIRST REPORT".
Can I have some guidance please ?
Rgds