The documentation portal is undergoing maintenance. During this period, you can't create any new user credentials. If you do, you'll need to recreate the user credential after the maintenance period ends.
You can enter an expression manually or use the point-and-click method. Select functions, ports, variables, and operators from the point-and-click interface to minimize errors when you build expressions. The maximum number of characters you can include in an expression is 32,767.