To add a custom item field in My Account

1. Go to transaction_line_views_cell_actionable.tpl template.
2. Add the following code to the template wherever you want the field to appear:

{{model.item.custitem_my_custom_field_id}}
custitem_my_custom_field_id is the ID of your custom item field

Save your code and test on a local server or deploy to NetSuite and activate as required.