Try the following:
Private Sub OpenFormButton_Click()
Dim stDocName As String
Dim stLinkCriteria As String
Dim stPolicy as Variant
stDocName = "frmNameofForm"
stPolicy = InpuBox "What is Policy Number?"
stLinkCriteria = "stPolicy =" & Me![PolicyNumber]
DoCmd.OpenForm stDocName, , , stLinkCriteria
End Sub