Hello,

Add new email template with your content using the xml. add the template M2O field into the note.note object

inherit the note form view and add that M2O field into the note form.

class note_note(models.Model):
    _inherit = 'note.note'
    
    template_id = fields.Many2one('mail.template', string="Template")
     
    @api.onchange('template_id')
    def onchange_tempalte(self):
       temp_id = self.env.ref('yourmodule.email_temp_id') # where email_temp_id is email template which you add using the xml
       body_html = self.env['mail.template'].sudo().render_template(
                    temp_id.body_html, 'note.note', self.id)
       self.memo = body_html

thank you.