Hi Antonio,

the best way, I think, is to create a custom html module that you can assign to the 'My profile' menu item that you have created. The position of the module will depend on what the template you are using allows you to do.

You can also create a template override of the page, but in my opinion, it is more maintenance down the road. 

Ok, that sounds very manual intensive, no matter how it will be done...
I suppose one way to do this would be to create a user custom field, get that field in an override of the profile page and show it.

Ok, to get custom fields in the override, I would do something like:

$user = Factory::getApplication()->getIdentity();

$jFields = Joomla\Component\Fields\Administrator\Helper\FieldsHelper::getFields('com_users.user', $user, true); // Get all cutom fields of User context for the current user.

foreach($jFields as $jfield) { // make the fields easier to access
    $jFields[$jfield->name] = $jfield;
}

Then it is just a matter to do an echo $jFields['your-custom-field-name']->rawvalue; // or use ->value

to get the value.

Super :-)