src/Installer/Package/AbstractPackageInstaller.php
protected function getExtraParameterValueByKey($extraParameterKey, $defaultValue = null)
    {
        $extraParameters = $this->getPackage()->getExtra();

        $extraParameterValue = $extraParameters[static::EXTRA_PARAMETER_KEY_ROOT][$extraParameterKey] ?? null;

        return (!empty($extraParameterValue)) ? $extraParameterValue : $defaultValue;
    }
protected function getExtraParameterValueByKey($extraParameterKey, $defaultValue = null)
    {
        $extraParameters = $this->getPackage()->getExtra();

        $extraParameterValue = $extraParameters[static::EXTRA_PARAMETER_KEY_ROOT][$extraParameterKey] ?? null;

        return (!empty($extraParameterValue)) ? $extraParameterValue : $defaultValue;
    }
Search for parameter with specific key in "extra" composer configuration block

@param string $extraParameterKey
@param string $defaultValue

@return array|string|null