Hi, I'm wanting to copy the value of a field from the Inventory Adjustment to a Fixed Asset that is created through the Asset Proposal / Manage Asset Proposal process. Can anyone point me in the general direction on how to achieve this.

You need to setup Transaction Field mapping for FAM. You need the corresponding field then on the FAM records as well. Go to Fixed Assets > Setup > Transaction Field Map > New. Official documentation is pretty clear on what is needed: https://docs.oracle.com/en/cloud/saas/netsuite/ns-online-help/section_1501725267.html

using script

Thanks @Jussi Penttinen that worked well! So easy when you know how!