Pre-Populate Dropdown in WebForm from link
hook_form_alter() or hook_form_form_id_alter() is what you're looking for here.

First your link how ever it's constructed should have a query parameter kind of like "/webformurl?product=a". This would be a link back to your webform. 

In one of the hook form alters, you'd check for the value in the url if it doesn't exist do nothing. If it does change the default value for the form field.  

If you're not sure how to alter the field on your form maybe check out this link for additional details - Programmatically modifying a webform with hook_form_alter()