i declared a new sequence field  in a relation of one2many of an existing module (event.event) 

like this 

    class event_trajet_line(models.Model):

    _inherit = "event.event"

    trajet = fields.One2many('event.trajet','trajet_id', string="Trajets")

    class event_trajets(models.Model):

    _name = "event.trajet"

    _description = "Trajet de la mission"

    name_id = fields.Char(string='Trajet Reference', required=True, copy=False, readonly=True, index=True, default=lambda self: _('New'))

    @api.model 

    def create(self, vals):

        if vals.get('name_id', _('New')) == _('New'):

            vals['name_id'] = self.env['ir.sequence'].next_by_code('trajet.order') or _('New')

        result = super(event_trajets, self).create(vals)

        return result 

and i declared then the sequence code and its suffix, prefix in a new xml file on a new directory data 

and i call the name of the directory to the _manifest_.py 

and the field works just fine, 

but i need to add it to the field 'name' of the form id "event.view_event_form" 
is there any solution to aproche this ? 

NP i tried this xml structure but it dosen work 

    <xpath expr="/form/sheet/div[@class='oe_title']" position="replace">

                    <div class="oe_title">

                            <label for="name" class="oe_edit_only"/>

                            <h1><field name="name" placeholder="Event Name"/><field name="trajet">          <field name="name_id"/></field></h1>

                            <field name="is_online" groups="base.group_no_one"/>

                            <label for="is_online" string="Online" groups="base.group_no_one"/>

                        </div>

                    </xpath>