Modules & VBA :: Function - Can A String Be Optional Variable?

I am amending some Code I found online for an audit table, I need to store additional information in the table that is associated with some forms but not others.

I have researched about putting optional variables in, but I read this only works with the type VARIANT.

Is there a way to make a string optional as my fields contain text?

here is my code so far:

Sub AuditChanges(IDField As String, UserAction As String, Optional UserID As String, Optional DeviceID As String, Optional SimID As String)
On Error GoTo AuditChanges_Err
Dim cnn As ADODB.Connection
Dim rst As ADODB.Recordset
Dim ctl As Control
Dim datTimeCheck As Date

[Code]...