From v7.3, you can follow the steps below to assign the necessary user permission to use the Metadata Search. 

Step 1, assign the managed (existing) Metadata Search Permissions permission set.  [Required]

  1. Go to Setup > Permission Sets > click on “Metadata Search Permissions” > click on the Manage assignments button.

  2. Assign this permission set to desired users.

Step 2, Create (and assign) a new permission set labeled Metadata Search Permissions (with API Enabled).  [Required]


Please create a “Metadata Search Permissions (with API Enabled)” permission set and assign this permission set to desired users.

  1. Go to Setup > Permission Sets > click on Create button > enter label as “Metadata Search Permissions (with API Enabled)” > Save.

           

  1. Click on “System Permissions” > Edit > select “API Enabled” & “Lightning Experience User” checkbox > Save.

       

           


  1. For users needing search access to set up and config entities like Flows, workflow rules, etc., check the “View Setup and Configuration” checkbox > Save. The “View Roles and Role Hierarchy” permission will be automatically checked.

  2. For users needing search access to Buttons and Links, check the "Customize Application" checkbox > Save.

  3. Now assign this new permission set to desired users.


Step 3,  Enable Lightning Web Security [Required]

  1. Please enable Lightning Web Security.
  2. Go To Setup -> Session Settings -> Lightning Web Security
  3. Check/Tick Use Lightning Web Security for Lightning web components  

Step 4, Add remote site setting for classic UI [Required for classic UI]

  1. Go to the Salesforce classic UI and open the Metadata Search tab.

  2. Could you copy the URL as shown below?

       

  1. Go to Setup > Remote Site Settings > click New Remote Site > Add remote site name and remote site URL with copied URL in step 2 above.

       

Please Note:

  1. View Setup and Config permission are required to search the following metadata entities:

    1. ExternalString (Custom Labels)

    2. FieldSet

    3. Flow

    4. LightingWebComponent

    5. Layout

    6. CustomTab

    7. ValidationRule

    8. WorkflowAlert

    9. WorkflowFieldUpdate

    10. WorkflowOutboundMessage

    11. WorkflowRule

    12. WorkflowTask

  2. Customize Application permission is required to search Buttons and Links. 
  3. API Enabled permission is required irrespective of the search (metadata) entity.