This walkthrough goes over different ways that you can add search functionality to your Write.as blog. It will be updated with more details and methods as they come along.
Feel free to leave questions, feedback, and suggestions about other ways to implement search functionality into Write.as.
Embedded Search Bar
DuckDuckGo has an embeddable search box that you can add to a dedicated search post of your blog. All you need to do is go here and create your DDG search box - be sure you add your Write.as blog url in the Site search field and change the Prefill. Once ready, copy the updated iframe, put it into the dedicated search post for your Write.as blog, and publish & pin the search post.
Write.as supports hashtags for posts. When you click on a hashtag, it will go to a page with a list of posts on your blog with that same hashtag (learn more here). What you can do is have a dedicated post that lists out the common hashtags you use in your blog. That way your readers can search your blog by topic rather than by words found in your posts.