There are bunches of videos that cover custom numbering but they kind of build on each other.

Sequential Numbers
Customer Codes
Sequential Annual Coding

To see if it already exists see
Access Expert 23 Composite Key
and
Prevent Duplicates

Guys, then is there a  way to make the VBA Code check to see if there an Similar "Contract  Number" exist already in the Table. If so, can it then make the New Contract Number use the "Existing Contract Number with the Same Prefix and Customer Code" go up to the Next Number?

Depends on what you mean by similar.  If you use a wildcard criteria like "CON*NABEL", it still requires exact matches: the first 3 letters must be CON and last 5 must be NABEL.  If by "similar," you mean "fuzzy" criteria or phrases, you are probably venturing into AI territory.  AI mimics human senses and thinking, which are often "fuzzy".  And fuzzy logic is difficult to program for, hence the need for advanced logic, like AI.  Here is a "fuzzy" example that is hard for a machine to read but should be easy for most humans to recognize:

"It deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae; the rset can be a toatl mses and you can sitll raed it wouthit porbelm; tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe."

Even though it is littered with misspelled words, you are able to read the above with relative ease because, as the above sentence says, humans don't read every letter by itself.  Humans use a kind of "fuzzy" logic when they read: if a misspelled word looks SIMILAR to the correct word, no problem.  This kind of fuzzy logic is hard to program.

Kevin, the Prefix is "CON" and the Customer Code would be like: "NABEL". So, I am looking for the SAME "Prefix and Customer Code that exist in the Table already, if so, add + 1 the the Number.

I get this Code to generate the Contract ID Number but it won't go up to the Next Number.


Function GenerateContractNum() As String

    Dim prefix As String
    Dim clientCode As String
    Dim newContractNumber As String
    Dim existingContractNumber As String
    Dim nextNumber As String
    
    prefix = "CON"
    clientCode = DLookup("ClientIDCode", "ClientT", "ClientID=" & Forms!ContractF2!CboClient.Column(0))
    
    newContractNumber = prefix & "-" & clientCode & "-" & "01"

    existingContractNumber = Nz(DLookup("ContractIDNumber", "ContractT", "ContractIDNumber='" & newContractNumber & "'"), 0)
    
    If existingContractNumber <> "" Then
        nextNumber = Val(Mid(existingContractNumber, Len(prefix) + 2)) + 1
        newContractNumber = prefix & "-" & clientCode & "-" & Format(nextNumber + 1, "00")
    End If
    
    GenerateContractNum = newContractNumber

End Function

Private Sub Form_BeforeUpdate(Cancel As Integer)

    ContractIDNumber = GenerateContractNum()
    
End Sub

Is this the sort of functionality you are looking for (see screenshot)?
You may need to modify the code for your specific needs.
Give it a go and let us know if it's what you need.

Kevin, you are awesome. The code work perfectly with the examples you displayed. Thank you so much for your help.