See how it is done in account_invoice.py:

    @api.multi
    def action_invoice_sent(self):
        """ Open a window to compose an email, with the edi invoice template
            message loaded by default
        """
        assert len(self) == 1, 'This option should only be used for a single id at a time.'
        template = self.env.ref('account.email_template_edi_invoice', False)
        compose_form = self.env.ref('mail.email_compose_message_wizard_form', False)
        ctx = dict(
            default_model='account.invoice',
            default_res_id=self.id,
            default_use_template=bool(template),
            default_template_id=template.id,
            default_composition_mode='comment',
            mark_invoice_as_sent=True,
        )
        return {
            'name': _('Compose Email'),
            'type': 'ir.actions.act_window',
            'view_type': 'form',
            'view_mode': 'form',
            'res_model': 'mail.compose.message',
            'views': [(compose_form.id, 'form')],
            'view_id': compose_form.id,
            'target': 'new',
            'context': ctx,
        }

or in sale.py

class mail_compose_message(osv.Model):
    _inherit = 'mail.compose.message'

    def send_mail(self, cr, uid, ids, context=None):
        context = context or {}
        if context.get('default_model') == 'sale.order' and context.get('default_res_id') and context.get('mark_so_as_sent'):
            context = dict(context, mail_post_autofollow=True)
            self.pool.get('sale.order').signal_workflow(cr, uid, [context['default_res_id']], 'quotation_sent')
        return super(mail_compose_message, self).send_mail(cr, uid, ids, context=context)