Find a Worksheet in the current project from a book sheet range string, like [book2]sheet2. If not find, add a new one. If the current project contain book2, then add a sheet named sheet2 in book2. If not, create a new worksheet in a new created workbook using the specified book name and sheet name.
You can use an empty string to get the active sheet from the active book, but if no active book or active book is not a workbook, then the output refnum for Origin.Worksheet will be invalid.