Silk, Web Weaver

If any permanent has an ability that triggers when the permanent you return leaves the battlefield, that ability will be put on the stack after you finish casting the spell, and it will resolve before that spell.

Web-slinging represents an alternative cost. If another effect allows you to play a card for a different alternative cost, including "without paying its mana cost," you can't also choose to pay its web-slinging cost. If any additional costs from other sources are applied, you must pay those.

You must follow all normal timing rules when casting a spell with web-slinging. For creatures, that usually means you must cast it during a main phase of your turn while the stack is empty.