Make any Salesforce Classic field searchable through basic sidebar search

This article explains how to make a custom field searchable through the Salesforce Classic basic sidebar search by creating a custom text field with the External ID option enabled and using a workflow rule to populate it.
Example use cases:

• A custom object uses an auto-number record format and you want users to search by a different field rather than the record number.
• You want all Opportunities related to a specific Account Name to appear in a basic sidebar search.

Note: A custom field without External ID can only be searched using the Advanced Search, not the basic sidebar search.

Steps to Enable Sidebar Search via External ID Field

Step 1: Create a custom text External ID field

1. Create a custom text field on the object (see Create a custom text field for full instructions).
2. In the field settings, enable the External ID option.
3. Since this field is used for searching purposes only, do not add this field to the Page Layout.

Step 2: Create a workflow rule to populate the External ID field
Create a workflow rule that updates the custom text External ID field with the value you want to search for.See Create a workflow rule  for full instructions.
Configure the workflow rule with the following settings:

• Evaluation criteria: Evaluate the rule when a record is created
• Rule criteria: Created Date — not equal to — null

Important Notes

• If the object does not have a tab (see Create a Custom Object Tab), it will not be searchable. The tab visibility must be set to either Default On or Default Off — it cannot be Hidden. If you do not want users to readily find the tab, set the visibility to Default Off and do not add it to any applications.
• Custom objects can be added to or removed from search by navigating to the object in Setup and enabling or disabling the Allow Search option.