Hi,

Like you said partners are already loaded to POS.You can access those in XML

Try the below code

<select>
    <option value=''>None</option>
    <t t-foreach='widget.pos.partners' t-as='partner'>
        <option t-att-value='partner.id'>
            <t t-esc='partner.name'/>
        </option>
    </t>
</select>

We will get the customers by looping widget.pos.partners. And you can use this in your required position where you need the dropdownlist.

Hope it helps