How do I make use of the order fields In Commerce?
The text that is submitted using your &lt;input&gt; field will display as expected with Your Reference: {{ cart.customerReference }}. 

However, I have a feeling that because the page is re-rendered with the inputs value as empty, when the /updateCart action is submitted again the blank value will override and erase the old value.

To solve this use the customerReference stored on the cart in the value attribute of your text &lt;input&gt;.

&lt;input ... value="{{ cart.customerReference|length ?? cart.customerReference }}"&gt;