In All conversations, other inboxes and folders, the search box is by default for the contacts. As soon as you tye the keywords, the search function will fetch the contact results.  


If you would like to search for any keyword in the messages, here is how you can do it. 

As soon as you type the word in the search, you will see a check box appearing. Please click to check the box to search the messages with the corresponding keyword. Once the box is checked, you will see the message search function kicking in searching for the messages with the relevant keyword in the search box.


Attaching screenshot for reference below;