Like most Shell() solutions, you need to embed quote marks around paths with space characters. It would go something like:
Sub Main()
  Dim exe$, mtb$, q$, s$
  
  exe = "c:\Program Files (x86)\Minitab\Minitab 18\mtb.exe"
  mtb = "C:\Users\Ken\Downloads\CementData.mtb"
  q = """"
  
  If Dir(mtb) = "" Then
    MsgBox "mtb's path does not exist.", vbCritical, "Macro Ending"
    Exit Sub
  End If
  If Dir(exe) = "" Then
    MsgBox "mtb.exe's path does not exist.", vbCritical, "Macro Ending"
    Exit Sub
  End If
  
  s = q & exe & q & " " & q & mtb & q & " 1"
  'Debug.Print s
  '"c:\Program Files (x86)\Minitab\Minitab 18\mtb.exe" "C:\Users\Ken\Downloads\CementData.mtb" 1
  Shell s, vbNormalFocus
End Sub


'CementData.mtb
'# https://support.minitab.com/en-us/datasets/regression-data-sets/cement-composition-data/
'Retrieve "C:\Users\Ken\Downloads\CementData.MTW"
'HISTOGRAM C1 C2;
'    YFrequency;
'    Bar;
'    Panel.