How to find the PO Number field when entering an order?

[Uni-Trend]

Hello,

In the Edit Order Invoice template in my store, there's the following code already in place:
{%- if po_number %}
<tr>
<td class="po-number__cell">
<span class="po-number__text">
PO number #{{ po_number }}
</span>
</td>
</tr>
{%- endif %}

Where does one find the field PO Number when entering an order? I have some customers who will sending in orders via PO. I would like to enter their PO number into their order when I Create Order. Otherwise, I would like just make an order Metafield called PO_Number and display it on their order. 

Any help would be great. 

Thanks!

Pete

[Uni-Trend]

bumping to find out where the native PO number function is. 

I have created a metafield for it, but would rather use something native, if possible. 

I am using the following in my email templates:

<tr>
<td class="po-number__cell">
<span class="po-number__text">
{{ order.metafields.custom.customer_po }}
</span>
</td>
</tr>

[idorulive]

I also want to know how to find the po_number is being set in the existing code.

[comac]

I would also like an answer to this if someone works it out please? My notification template has this in it by default 

{% if po_number %}
<tr>
<td>
<strong>PO number</strong><br/>
<p>#{{ po_number }}</p>
</td>
</tr>
{% endif %}