link_example.php

<?php

$build['your_key'] = [
  '#type' => 'link',
  '#title' => $this->t('My button'),
  '#url' => Url::fromRoute('your_route'),
  '#attributes' => [
    'class' => ['button', 'use-ajax'],
    'data-dialog-type' => 'modal',
    'data-dialog-options' => json_encode([
      'height' => '500',
      'width' => '70%',
      'title' => $this->t('My form'),
    ]),
  ],
  '#limit_validation_errors' => [],
];