Field tag

Hi there, I have created an extra usefield named ‘KLANTNUMMER’ with the label ‘klantnummer’. How can I use this in e.g. my Woocommerce PDF Invoice plugin? This plugin allows me to add additional fields by including them in content block between bracket {{field}}. What is the ‘code’ or ‘tag’ that I have to use to get the value of the extra user field into the Pdf invoice? I have tried {{klantnummer}} but this does not work.