Basically, there is no difference when using custom user functions in Type (.CS) and Declarative (.TRDP) reports.
The custom function could be any public static method
that resides in a loaded assembly
can be invoked from an expression by its fully qualified name, including the full namespace and the name of the type it belongs to, and specifying the necessary parameters in the braces - check User Functions
In order the custom function to be accessible in Standalone Designer, it would be necessary to register the corresponding assembly in the designer's config file, as explained in the Extending Report Designer
article. The custom functions appear under Functions
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