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.
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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
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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.
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Click copy and save your new custom URL to use as you need.