pos/pos_ticket_salesman_firstname/views/pos_config.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>


    <record id="view_pos_config_form" model="ir.ui.view">
        <field name="model">pos.config</field>
        <field name="inherit_id" ref="point_of_sale.pos_config_view_form"/>
        <field name="arch" type="xml">
            <xpath expr="//div[@id='iface_invoicing']" position="before">
                <div class="col-12 col-lg-6 o_setting_box" id="receipt_salesman_firstname">
                    <div class="o_setting_left_pane">
                        <field name="receipt_salesman_firstname"/>
                    </div>
                    <div class="o_setting_right_pane">
                        <label for="receipt_salesman_firstname"/>
                        <div class="text-muted">
                            On the receipt, prints only the salesman's firstname instead of the full name.
                        </div>
                    </div>
                </div>
            </xpath>
        </field>
    </record>

</odoo>