NetSuite uses Entity Search when Help Desk feature is enabled. Follow the steps enumerated to allow usage of Company search form instead of Entity search form in the Company field of Case Form:
1. Navigate to Setup > Company > Enable Features.
2. Click the CRM tab.
3. In the Support section, uncheck the Help Desk checkbox.
Note: This feature allows users to use case records for internal employee support as well as customer support.
4. Click Save.
5. Create a Company Search.
a. Navigate to Lists > Search > Saved Searches > New.
b. Select Company.
c. Set the required filters in the Criteria tab > Standard subtab,
d. Set the required fields in the Available Filters tab,
Filter (e.g. Contact : Category)
Show in Filter Region is checked
e. Check the My Preferred Search Form checkbox.
f. Click Save.