I've had to do something similar and it required creating a new label, since the product label has no reference to the picking when printed. You'll need to create a label for stock.picking and then iterate over the products/move lines in the transfer.

From the transfer, you'll be able to pull the contact (res.partner) info since it has a One2Many relationship to the contact.

I've made an ASN label in the past which takes a delivery order and prints a zpl label for each move line (so each product + lot combination) with some contact info. It shows how to tell the printer to start a new label and how to go over each product in a stock.picking

<t t-name="stock.label_ASN_template_view">
    <t t-foreach="docs" t-as="transfer">
        <t t-set="contact_name" t-value="transfer.partner_id.commercial_company_name"/>
        <t t-set="contact_street" t-value="transfer.partner_id.street"/>
        <t t-set="contact_city" t-value="transfer.partner_id.city"/>
        <t t-set="contact_zip" t-value="transfer.partner_id.zip"/>
        <t t-set="contact_state" t-value="transfer.partner_id.state_id.code"/>
        <t t-set="contact_country" t-value="transfer.partner_id.country_code"/>
        <t t-set="carrier" t-value="transfer.x_studio_transporteur"/>
        <t t-set="PO" t-value="transfer.sale_id.client_order_ref"/>
        <t t-translations="off">
            <t t-foreach="transfer.move_ids" t-as="move">
                <t t-foreach="move.move_line_ids" t-as="line">
            
^XACI28

^FX Border Definitions
^FX From field
^FO2,4
^GB254,203,2
^FS

^FX To field
^FO254,4
^GB558,203,2 
^FS

^FX Postal Code field
^FO2,203
^GB508,206,2
^FS

^FX Carrier info field
^FO508,203
^GB304,206,2
^FS

^FX Product info field
^FO2,409
^GB812,406,2
^FS

^FX MAN Segment field
^FO2,815
^GB812,403,2
^FS

^FX Data definitions
^FX From field
^FO10,16
^A0N,26,26^FB200,1^FDFrom/DE:
^FS

^FO20,50
^A0N,26,26^FB225,5^FDImpress Foods Inc. 1910 rue du Sanctuaire, Quebec, QC G1E 3L2
^FS

^FX To field
^FO264,16
^A0N,26,26^FB200,1^FDTo/A:
^FS

^FO284,50
^A0N,26,26^FB300,5^FD<t t-out="contact_name"/>. <t t-out="contact_street"/>, <t t-out="contact_city"/>, <t t-out="contact_state"/> <t t-out="contact_zip"/>
^FS

^FX Postal code field
^FO10,213
^A0N,26,26^FB500,1^FDShip to Postal Code:
^FS

^FO15,243
^A0N,26,26^FB500,1^FD(420)<t t-out="contact_zip"/>
^FS

^FO90,288
^BCN,100,N,N,Y^FD420<t t-out="contact_zip"/>
^FS

^FX Carrier Field
^FO518,213
^A0N,26,26^FB500,1^FDCarrier:
^FS

^FO518,243
^A0N,26,26^FB500,1^FD<t t-out="carrier"/>
^FS

^FO518,273
^A0N,26,26^FB500,1^FDPro: 12345
^FS

^FO518,303
^A0N,26,26^FB500,1^FDB/L: 123456789
^FS

^FX Product Field
^FO20,439
^A0N,40,40^FB500,1^FDPO: <t t-out="PO"/>
^FS

^FO20,489
^A0N,40,40^FB500,1^FDGTIN: <t t-out="line.product_barcode"/>
^FS

^FO500,489
^A0N,40,40^FB500,1^FDLOT: <t t-out="line.lot_id.display_name"/>
^FS

^FO20,539
^A0N,40,40^FB800,2^FDItem Description: <t t-raw="line.product_id.display_name"/>
^FS

^FO130,639
^A0N,40,40^FB800,1^FDCase QTY: <t t-out="line.quantity"/>
^FS

^FO130,689
^A0N,40,40^FB800,1^FDProduct Net Weight (KG):
^FS

^FO130,739
^A0N,40,40^FB800,1^FDCode Date: <t t-esc="line.lot_id.use_date.strftime('%Y-%m-%d')"/>
^FS

^FX MAN Segment
^FO10,822
^A0N,26,26^FB500,1^FDMAN Segment (SSCC-18):
^FS
^FO110,900
^BCN,225,Y,Y,Y,A^FD(00)003001234000000001

^XZ 
                </t>
            </t>
        </t>
    </t>
</t>

Just to clarify, we're printing from the delivery ticked so the sales order number would be the Source Document field.