How to set default field value from xml code in odoo?

In sales order module I have created a new customised form view, in sale.py file I have created a new field 'is_sample'

'is_sample': fields.boolean("Specimen Order", store=False),

I want to set its default value from xml code so that it doesn't get affected in default form view. I have tried in four ways,

1)<field name="is_sample" eval="True"/>
2)<field name="is_sample" domain="[('is_sample','=',True)]"/>
3)<record id="action_specimen_orders" model="ir.actions.act_window">
        <field name="type">ir.actions.act_window</field>
        <field name="context">{'is_sample': 'True'}</field>
       ...
4)<record id="action_specimen_orders" model="ir.actions.act_window">
        <field name="type">ir.actions.act_window</field>
        <field name="domain">[('is_sample','=','True')]</field>
       ...