Hi,

For opening the form view when clicking on the add a product you have to remove the editable="bottom" from the sale order line tree view first. Once you do this, a form view will get opened.

To add new fields to that form view:

in the python:

class SaleOrderLine(models.Model):
    _inherit = 'sale.order.line'

    test = fields.Char(string='Test')

In the XML,

<record id="sale_order_view_inherit" model="ir.ui.view">
    <field name="name">sale.order.view</field>
    <field name="model">sale.order</field>
    <field name="inherit_id" ref="sale.view_order_form"/>
    <field name="arch" type="xml">
        <xpath expr="//field[@name='order_line']/form/group/group/field[@name='price_unit']" position="after">
            <field name="test"/>
        </xpath>
    </field>
</record>

You will see many other fields in the form, you can hide unnecessary fields depending on your use case.

Thanks