I cannot find this possibility in Writer and I am not sure if it really exists.
Some eBooks formats require free open pages to choose font size and therefore do not allow to upload an index with page numbers.
See: Wiki Help · Entries (table of contents) · Structure and formatting
The Structure line defines how the entries in the index are composed. To change the appearance of an entry you can enter codes or text in the empty boxes on this line. You can also click in an empty box or on a code, and then click a code button.
• To delete a code from the Structure line, click the code, and then press the Delete key.
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