How to Customize Search Navigation in SharePoint Online?

Requirement: Customize search drop down by adding a new search scope in SharePoint Online.

How to Customize the Search Navigation in SharePoint Online?
The default search drop-down has below options:

1. Everything
2. People
3. Conversations
4. This Site

We need a new custom scope (drop down option) to the search drop down to be added. This document outlines How to add a link to search drop down to redirect to the custom search center.

Prerequisites: Assuming the custom search center site is ready!

SharePoint Online: Configure Search Navigation

Here is how to add a link to search drop down - Step by Step:

1. Login to the SharePoint Online site as a Site collection Administrator. Click on Settings gear and then select Site Settings link from the menu.
2. In Site Settings Page, Click on "Search Settings" under Search group.
3. In Search Settings page, scroll to “Configure Search Navigation” Section. Click on “Add Link” Hyperlink
   
4. Enter the Title for new search scope and URL (E.g. https://crescent.sharepoint.com/search/Pages/corporatedirectory.aspx) of the search center. Click on "OK" to close the popup page. 
5. You can move up/down, adjust other options in the drop down. Click on “OK” to save your changes in the search settings page. Now, you should see the new option in the search drop-down! 
   

Enter a search query and make sure the new search center results are served!