After you have created a short link, you should be able to find your short link(s) under the "Short Links" dashboard.Beside the short link you would like to customize, click on the 3-dot (..) icon.
Select "Custom Link" from the drop-down menu.
Enter in your custom short link. (Please note: your custom short link should contain at least 4 alpha-numeric characters, no special characters or spaces are allowed. If you are unable to create a custom short link, it means that the custom short link has already been taken by another user.)