Create your own tokens with a hook

You'll want to look at hook_tokens and hook_token_info.

Those will let you define the tokens and define the token replacements.

There is also a patch you could apply which exposes all custom entity fields as tokens. If what you want to create a token for is a registrant field itself and not something custom then take a look at

✨ Consistent tokens support across all entities and fields Needs work

@pixiekat Thank you, it worked:

function MY_MODULE_token_info() {

    $registrant = []; 

    $registrant['id'] = [
    'name' => t('Registrant ID'),
    'description' => t('Get the ID of the registrant.'),
    ];

    $registrant['uuid'] = [
    'name' => t('Registrant UUID'),
    'description' => t('Get the UUID of the registrant.'),
    ];
    
    return $registrant;

}

function MY_MODULE_tokens($type, $tokens, array $data, array $options, \Drupal\Core\Render\BubbleableMetadata $bubbleable_metadata) {

    $replacements = [];

    $registrant = !empty($data['registrant']) ? $data['registrant'] : NULL;    
    
    if ($type == 'registrant' && !empty($data['registrant'])) {
        foreach ($tokens as $name => $original) {
            switch ($name) {
                case 'id':
                $replacements[$original] = $registrant->id->value;
                break;    

                case 'uuid':
                $replacements[$original] = $registrant->uuid->value;
                break;  
            }
        }
    }
    
    return $replacements;
}