As indicated in my previous reply, you can use the 'mark all' indexing option. Just make sure the content you select for the initial index entry is in the format you want it to appear as.
Alternatively, you could create the Index using a concordance file. That way, the format of the index entries will be as defined in the concordance file.
The Index concordance document is essentially a two-column table in which the texts to be indexed are in the first column and the form of the Index entries are in the second column. Done properly, the process is effective, straightforward, and efficient. However, for a cautionary note on what can happen if you're indiscriminate about what you index this way, see:
https://wordmvp.com/FAQs/Formatting/CreateIndex.htm
Once the entries are marked, see:
https://support.office.com/en-us/art...2-cda9d14bf073