Navigate to Admin --> Users.
Select the External Users tab.
Search for the external user that you assigned an item to.
Next to that external user's email address, click the pencil icon.
In the pop-up modal, select the Access tab.
Find the Record in question and click on the 'copy' icon to copy the token to your clipboard.
You can then send that token (URL) to the external user, and they should be able to access the Record from there.
Additionally, you can access the token URL through the record itself.
Navigate to the record that is assigned to the external User.
Click on the “Copy external link” button next to the name of the current assignee.
The link is now copied and can be pasted into any message or email that you choose to send the external user.
Note: In order to send out the URL this way, you must possess the Admin Entitlement.