It's possible the field has a different display name than the field name. It's also possible that the GUI has a label that is not the same as the field name.
If "External Script Name" is the display name, then you can find the field name in a couple ways.
If you have admin permissions: Launch the Admin Client, and navigate to Workflows and Documents > Fields. Add the Display Name to the columns. Search for fields containing "External Script Name".
If you don't have admin permissions: Run "im fields --fields=name,displayname --noasadmin >> fieldlist.txt" from the CLI. Search the text field for "External Script Name", and see what shows as the name.
If "External Script Name" isn't the display name:
From the Admin Client, check the issue presentation template to see what is defined.
From CLI, view the item using "im viewissue itemid" . The display name for "External Script Name" will be used in the output. You can then use the im fields command above to find the name.