Queries :: Passing Parameter To Select Query?

I have form with a button on it that launches a parameter-based Select query (which served as the source for a report). I didn't have any validation measure in place, so if the User supplied a bogus value, a blank report was generated. While not technically an error, it would seem more polished to generate a warning message if the User supplies a bad query value and prompt them to re-enter.

Having read other posts along these lines, I've added an unbound text box to the form which the User fills in first before clicking the button. When the button is clicked, it executes code that uses the DCount function to make sure the text box value is in the source table, and if it is then it runs the parameter-based Select query. My question is how to pass the value in the text box to the query as a parameter. Below is a sanitized version of the code that I've generated so far.

Private Sub SingleItemRptB_Click()
If DCount("[FieldName]", "[Table]", "[FieldName]=[TextBoxValue]") = 0 Then
MsgBox "Item not in database. Please check value and re-enter."
Else
DoCmd.OpenQuery "SingleItemQ", acViewNormal, acReadOnly
End If
End Sub