You could put some code using the Dlookup function on the OnCurrent Event of the form like this:
Private Sub Form_Current()
Me![ControlOnForm] = DLookup("[TableFieldName]", "TableName", "[PrimaryKey] = " & Chr(34) & Forms![FormName]![PrimaryKeyOnForm] & Chr(34))
Me![ControlOnForm] = DLookup("[TableFieldName]", "TableName", "[PrimaryKey] = " & Forms![FormName]![PrimaryKeyOnForm]
End Sub
I gave one example for matching a string, the other to match a numeric.