1. Create a custom list of Alphabets with values from A-Z.
a. Navigate to Customization > Lists, Records & Fields > Lists > New
b. Enter Name (Example: Alphabets)
c. Enter Values from A to Z
d. Click Save.
2. Create a Multiple Select-type custom field to source the list of Alphabets
a. On the Edit Custom Record Type page, click New Field under Fields tab
b. Enter Field Name (Example: Alphabets)
c. Select the Type = Multiple Select
d. Set List/Record = Alphabets (custom list created in A)
e. Click Save.
3. Navigate to Home dashboard tab> Set Preferences link > General tab > Optimizing NetSuite section.
4. Change Maximum Entries in Dropdowns value to 0
5. Click Save.
6. View a custom record where the custom Multiple Select field is applied to.
As described on the Help article Using Popup and Dropdown Lists, when Maximum Entries in Dropdown is less than the number of values of a Multiple Select field, Popup Multi Select List will be available.
When Maximum Entries in Dropdown is more than the number of values of the Multiple Select field, the values will be listed in the input box.