Symfony 6 - EasyAdmin - Custom form type canvas

I use Symfony 6 with EasyAdmin, I've been trying for several hours to understand how form types work in Symfony, and more specifically how to set up a custom form field, but I can't find a simple, comprehensive example. Perhaps one of you could help me?

My need is simple: I have a "signatureClient" field for which I want to generate a <canvas id="signatureClient" columns="300" height="100"></canvas> tag, for the purpose of creating a digital signature.

I created a SignatureField class.

<?php

namespace App\EasyAdmin;

use EasyCorp\Bundle\EasyAdminBundle\Contracts\Field\FieldInterface;
use EasyCorp\Bundle\EasyAdminBundle\Field\FieldTrait;
use App\Form\CanvasType;

class SignatureField implements FieldInterface
{
    use FieldTrait;

    public static function new(string $propertyName, ?string $label = null)
    {
        return (new self())
            ->setProperty($propertyName)
            ->setLabel($label)
            ->setFormType(CanvasType::class)
            ;
    }
}

In my controller,

SignatureField::new('signatureClient')

Then a CanvasType class, for which i've chosen the textarea parent, and the prefix "canvas":

<?php

namespace App\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormView;
use Symfony\Component\Form\Extension\Core\Type\TextareaType;

class CanvasType extends AbstractType
{
    public function getParent(): ?string
    {
        return TextareaType::class;
    }

    public function getBlockPrefix(): string
    {
        return 'canvas';
    }
}

My custom_types.html.twig

{# templates/admin/form/custom_types.html.twig #}

{% form_theme form 'form.html.twig' %}
{% block _InterventionTechnique_signatureClient_widget %}

    <canvas id="{{ field.name}}" width="300" height="100">{{ field.value}}</canvas> 
{% endblock %}

The result in my form:

 <textarea id="InterventionTechnique_signatureClient" name="InterventionTechnique[signatureClient]" class="form-control"></textarea>

The link with the canvas template is not working...

Could one of you help me?

Thank you very much.