First you will need to get the URL to the product or collection you want to create a custom URL to. Navigate to the product or collection and copy the URL as shown in the image below.
Now login to your affiliate portal Here. You will land on your Dashboard page and click on Create link to a specific page as shown in the image below
Past the URL for the product or collection that you copied earlier to the Enter destination page URL field and click Create Link as shown in the image below.
Click copy and save your new custom URL to use as you need.