Installing extensions in composer based TYPO3 installations is not possible via the TYPO3 backend.

Trying so will yield the following error message:

Composer mode

The system is set to composer mode. Please notice that it is not possible to add an extension with the Extension Manager. You have to use composer to add further extensions to your system.

This tutorial assumes you're developing locally with docker/composer.

In our example we're adding the extension "powermail".

Find out the extension's composer path

You can usually get the composer path from the extension repository. Just search for the extension and look for "composer support".

For powermail, head to

https://extensions.typo3.org/extension/powermail

The composer path is "composer req in2code/powermail".

Find out which version of the extension supports your TYPO3 version

Make sure that the version of the extension you're about to install is compatible for your TYPO3. If you want to install powermail in TYPO3 9.5 you can't install the newest version but need to install Version 7.4.

Add the extension to composer requirements

Now add the extension to composer requirements:

docker-compose run composer --ignore-platform-reqs require in2code/powermail ^7.4

To explain:

• --ignore-platform-reqs
  ignores platform requirements - so if you don't have gd installed in the composer image installation does not fail
• in2code/powermail
  this is the path where the composer version of the extension is maintained
• ^7.4
  specifies the version we want to install (7.4.x)

Update you composer installation

docker-compose run composer update

After this step the extension is installed and you can activate it.

Deployment

If your docker/composer setup is configured for automatic deployment you don't need to do anything. Pushing to the respective branches builds the TYPO3 as per the composer requirements and pushes the build to staging / production environment.