@Seth Duda - can you give us some context here - so, you are trying to achieve this by using a client script, right? Normally all these additional fields populate on postSourcing of the item column field, i.e. the item has been just selected by the user

I'm trying to add a line via a server side script (not client side). I would like the tax code to source automatically, but it appears NetSuite will only do this when an item is selected via the UI.

NetSuite does not perform "sourcing" steps when server side script is executed... I realized that for all record types... NetSuite has a lot client scripts on UI side... For example, use chrome (or any other browser) and try to inspect html components... You will find a lot of scripts for "onchange" events 😁. I am really disappointed with that...

Yeah, I think in this instance it's going to require setting fields on the client side to work around this... I agree, very disappointing. I haven't found any other option.