Edit the product
Under the editor, click where you want to place the Lazada/Shopee button
Click the "Add Media" button
Select the button image and click "Insert into product"
Select the newly-added button, then click the "link" button
Paste the link, then click the gear icon to configure
Tick to enable the "Open link in a new tab" option, then click "update"
Finalize your edit by clicking the "Update" button
To view, click the "View Product" link.