This thread is resolved. Here is a description of the problem and solution.
Problem:
How to add custom search to author page Solution:
To create author archive with custom search option you need to set the archive purpose to "Display the archive as custom search" and you will see the Filter editor available with your archive.
My "author" pages currently look like this: hidden link
Aside from the info at the top posts belonging to the author are also displayed.
What I want to try and accomplish is to create a custom search with 2 fields, keyword, and a category drop-down. the search will be displayed on the author's page.
Is it possible to create a custom search that will only search the listing of that specific author's posts?
Well - did you created author's archive from: Toolset => WordPress archives?
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I would also eventually need to request temporary access (WP-Admin and FTP) to your site. Preferably to a test site where the problem has been replicated if possible in order to be of better help and check if some configurations might need to be changed.
I would additionally need your permission to de- and re-activate Plugins and the Theme, and to change configurations on the site. This is also a reason the backup is really important. If you agree to this, please use the form fields I have enabled below to provide temporary access details (wp-admin and FTP).
I have set the next reply to private which means only you and I have access to it.
Well - you need to set the correct archive purpose - please check following screenshot:
=> hidden link
I've set the archive purpose to "Display the archive as custom search" and now you will see the Filter editor available with your archive. You should remove all filters from your loop output editor and add it to Filter editor and it should work.