How can I set the value of Custom Form on a transaction record when the customer (entity) is preset? I need this to work when the transaction is created by other scripts so I can't use a client script

best choice is probably a before submit user event script

If you control the other scripts creating the transaction, you could also set

customform

with the

defaultvalues

option in

record.create()

(or the 1.0 equivalent if it's a 1.0 script).