You will need to create the T1059 folder in Atomics directory. Save this as a new yaml in that dir. A second folder under T1059 will be src. place the au3 file there.

automsgbox.au3

MsgBox(0, "Atomic Message", "hello from Atomic Red Team")

T1059.yaml

attack_technique: T1059
display_name: Command and Scripting Interpreter
atomic_tests:
- name: AutoIt Message Box Test with Download and Extract
  description: |
    Downloads AutoIt to the temporary directory, extracts it, and executes an AutoIt script that shows a message box.
  supported_platforms:
    - windows
  input_arguments:
    autoit_script_path:
      description: The local path to the AutoIt script to execute
      type: Path
      default: "PathToAtomicsFolder\\T1059\\src\\automsgbox.au3"
  executor:
    name: powershell
    elevation_required: false
    command: |
      $ErrorActionPreference = 'Stop';
      $autoitExePath = "$env:TEMP\\autoit-v3\\install\\autoit3.exe";
      if (-not (Test-Path -Path $autoitExePath)) {
        iwr 'https://www.autoitscript.com/cgi-bin/getfile.pl?autoit3/autoit-v3.zip' -OutFile "$env:TEMP\\autoit-v3.zip";
        Expand-Archive -LiteralPath "$env:TEMP\\autoit-v3.zip" -DestinationPath "$env:TEMP\\autoit-v3";
      }
      Start-Process -FilePath $autoitExePath -ArgumentList (Resolve-Path "#{autoit_script_path}").Path;