Hi,

‘search_default_ref: ‘POS’ will not work here.

For filter default by POS you need to add new filter into the search view as follows :

<record id="filter_by_ref" model="ir.ui.view">
    <field name="name">Journal Items</field>
    <field name="model">account.move.line</field>
    <field name="inherit_id" ref="account.view_account_move_line_filter"/>
    <field name="arch" type="xml">
        <xpath expr="//search" position="inside">
            <filter string="POS Reference" name="pos_reference" domain="[('move_id.ref','ilike','POS%')]"/>
        </xpath>
    </field>
</record>

And now in the action you can pass the context as ‘search_default_pos_reference':1