Hello Joaquin Inamine,
I hope you are doing well

- Set up an Automated Action: Model Discuss Channel, Trigger On Creation, Filter [("channel_type","=","whatsapp")], Action Execute Python Code:

WELCOME_TEXT = "Thank you for contacting us, we'll get back to you shortly."
if record.channel_type == 'whatsapp':
    already = env['mail.message'].search_count([('model', '=', 'discuss.channel')('res_id', '=', record.id), ('author_id', '=', env.user.partner_id.id)])
    if not already:
        record.message_post(body=WELCOME_TEXT, message_type='whatsapp_message', subtype_xmlid='mail.mt_comment')

I hope this information helps you

Thanks & Regards,
Kunjan Patel