Once you have logged into your Mastodon account go to your account settings then down to development and create a new application,
once you've done this enter your app info into the below snippet into your /protected/config/common.php
file.
return [
// ...
'components' => [
// ...
'authClientCollection' => [
'clients' => [
// ...
'mastodon' => [
'class' => 'humhub\modules\user\authclient\Mastodon',
'clientId' => 'Your Mastodon App ID here',
'clientSecret' => 'Your Mastodon App Secret here',
'returnUrl' => 'http://yourdomain.com/user/auth/external?authclient=mastodon',
],
],
],
// ...
],
// ...
];