Default values on form

Hi,

I have a created a form to gather user information. Now I have the information stored on the database, I want to fill the form with the information the user has provided. I am trying to do it with twig variables on the "default" attribute of the form, just like this:
name: user-name
default: {{ userNameFromDatabase }}

I have a plugin that handles the information recovery and assignment to the twig vars:
public function onMarkdownInitialized(Event $event)
{
$this->grav['twig']->twig_vars['userNameFromDatabase'] = 'John';
}

It doesn't display anything. I have modified the system.yaml, pages - frontmatter - process_twig to true, but nothing happens neither.

Is there a proper way to print a form fulfilled with the user information?

Thanks,
xavi

I think the solution is here:
https://learn.getgrav.org/forms/blueprints/advanced-features#using-function-calls-data

But I don't understand the documentation, there are no more examples?