Open file Location - variable location

I am trying to code a VBA button where it will open a folder in windows explorer.

I can do it fine with the following code.

Sub Key_Register()
'
'
Dim Foldername As String
Foldername = "L:\KEY REGISTER\40025\"

Shell "C:\WINDOWS\explorer.exe """ & Foldername & "", vbNormalFocus
'
End Sub

The issue is, the location varies depending on the job number.

In the above case - it is Job 40025.

This number changes from job to job - it will always be numerical but not always 5 digits (it can be 6-digits).

I can put the job number in cell A1 where it will dynamically change.

How do I integrate this to the above code?