• PHP 7.1+
• Plesk 17.8 or higher
• One or more domains hosted with TransIP and API access

• Navigate to the releases tab: https://github.com/firstred/plesk-transip/releases/latest

• Download the extension

• Add the following to your panel.ini file:

  [ext-catalog]
  
  extensionUpload = true
  

  This makes sure that you can upload extension on the extension page.

• Navigate to the Extensions page, then My Extensions. Here you can upload the extension zip.

• TransIP should now be visible in your extension list.

The module needs your TransIP username and a private key to use for the API.
You can generate a new private key in your TransIP Config Panel. Make sure you whitelist your server's IP as well.

Install the dependencies first with composer. Unlike a regular composer package the vendor folder can be found in the subdirectory src/plib. This is the folder that eventually ends up on Plesk. Packing up the src/plib folder is enough to upload the module.
This is what an extension's file structure should look like: https://docs.plesk.com/en-US/onyx/extensions-guide/plesk-extensions-basics/extension-structure.71076/