Minimum 

  <form action="https://keilaserver.example.com/forms/form_id" method="post">
  <div
    <label for="contact_email">
      Email
    </label>
    <input id="contact_email" name="contact[email]" type="email"/>
  </div>
  <div
    <button>
      Submit
    </button>
  </div>
</form>

Custom Field 

  <form action="https://keilaserver.example.com/forms/form_id" method="post">
  <div>
    <label for="contact_email">
      Email
    </label>
    <input id="contact_email" name="contact[email]" type="email"/>
  </div>
  <div>
    <label for="contact_data_my_custom_field">
      My Custom Field (optional)
    </label>
    <input id="contact_data_my_custom_field" name="contact[data][my_custom_field]" type="text"/>
  </div>
  <div>
    <button>
      Submit
    </button>
  </div>
</form>

With Checkbox 

  <form action="https://keilaserver.example.com/forms/form_id" method="post">
  <div>
    <div>
      <label for="contact_data_interest_token1" style="display: inline-block;">Stardew Valley</label>
      <input checked id="contact_data_interest_token1" type="checkbox"
             name="contact[data][interests][token1]" />
      <label for="contact_data_interest_token2" style="display: inline-block;">Stardew Valley</label>
      <input checked id="contact_data_interest_token2" type="checkbox"
             name="contact[data][interests][token2]" />
    </div>
    <input placeholder="[email protected]" id="contact_email" name="contact[email]" type="email"/>
  </div>
  <div>
    <button class="button">
      Submit
    </button>
  </div>
</form>

Critical piece

    <div>
    <label for="contact_data_my_custom_field">
      My Custom Field (optional)
    </label>
    <input id="contact_data_my_custom_field" name="contact[data][my_custom_field]" type="text"/>
  </div>