I want to add an index for a word, in this example “myword”.
How can I automatically add the index on the new occurences of my word?
If I press the Insert button, and then I write “myword”, it will not be indexed.
To automatically include any occurrence of specific words or items in the index, you can use a concordance file. A concordance file essentially lists any terms that you want to have indexed wherever they appear.
See LibreOffice help and search for concordance for specific instructions on how to create and use concordance files.