Hi,
When you edit the product, under the Files area, you can use the "Add file" icon. That will open a popup where you can select "Specify file location" in the "Set file mode" setting, and below, in the "File path" input, enter the URL of your webinar page.
You can also set the "Download number limit" setting to 1 so that the download URL the customer will get after purchasing that product will only work once.
Now, please note that if the user has the direct URL to the webinar, he will be able to share that URL and access it as much as he wants since HikaShop has, by default, no control on the page where the webinar is available.
If you want that HikaShop restricts access to that webinar page, and provided it is on your website, you can use the hkshow tag of HikaShop around the area where the webinar is displayed on your website:
www.hikashop.com/home/blog/466-hikashop-4-4-1.html#tags
With this, you can ensure that the webinar displayed inside the tag will only be accessible if the current user has purchased the product.