Welcome to the Make community!
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IML functions is available for ALL custom apps— you just need to request for it to be enabled[1].
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Custom functions is an entirely separate feature for Enterprise organisations.
By the way, you can already use my module “Get Next Business Day (with holidays)”.
See My Custom App: save headaches and operation credits with these utility modules:
- Chunk Text
- Chunk Array
- Chunk Words
- Multiple Find & Replace
- Collection to Array/String List
- Execute JavaScript
- Estimate Tokens
- Calculate Difference Between Two Dates
- Get Next Business Day (with holidays)
- List Upcoming Dates of Day of Week
… and more
Hope this helps! If you are still having trouble, please provide more details.
— @samliew