In general, WooCommerce lets you add products to your cart through URLs. Once the Allow posting extra options as url parameters option is selected, you can add the custom fields created using the plugin to your cart directly through the URL.
For example:
https://yourwebsite.com/?add-to-cart=407&tshirtcolour=White&printtshirt=joe&thwepo_product_fields=tshirtcolour,printtshirt
Here, what we have done is, to the URL we have added the product Id (407), then added the custom fields and their value (&tshirtcolour=White&printtshirt=joe), which was created using the Extra Product Option for the WooCommerce plugin.
WooCommerce has a feature that allows you to add products to your cart via URLs. By enabling Allow posting extra options as url parameters option, add the custom fields that is created through the extra product option to your cart directly through the url.