Hi Kathy
You can follow this suggestion precisely being quoted thereafter, as added by Tom –
The option to place the search icon in the navigation can be found in “Appearance > Customize > Layout” – it’s the “Navigation Search” option.
Note – The above suggestion has been fetched from the forum topic link given by you
In case looking for precise walkthrough, Would suggest to go through this documentation excerpt – https://docs.generatepress.com/article/navigation-search/