Class not found

Yes, it is needed.
There are two options for calling a class in a namespace.

1
use Sendpulse\RestApi\ApiClient;
new ApiClient();

2
new \Sendpulse\RestApi\ApiClient();

The result will be the same.

Error "Class 'Sendpulse\RestApi\ApiClient' not found." means that the script cannot determine the location of the class.

There are three possible reasons:
1. The composer is not included
2. The library is not installed
3. The library is not installed correctly / the composer has not indexed the library in the boot files.

In the third case, you can try reindexing 'composer dump-autoload' or reinstalling the library.

If the installation and/or indexing was successful, then an entry about the path to the library should appear in the vendor/composer/autoload_psr4.php file.