Hello EM,

Please Try this. In this code, field will appear after the Source document(Origin).

<record id="stock_picking_form_view_inherit" model="ir.ui.view">

    <field name="name">stock.picking.form.view.inherit</field>

    <field name="model">stock.picking</field>

    <field name="inherit_id" ref="stock.view_picking_form"/>

    <field name="arch" type="xml">

        <xpath expr="/form/sheet/group/group/field[@name='origin']" position="after">

            <field name="custom_field"/>

        </xpath>

    </field>

</record>

Hope this will helps you.

Thanks,

It was actually a mistake I made in the xpath expression.

It is as easy as always:    

<xpath expr="//group[@string='Locations']" position="before">