The window closes faster than you think
Interest in giving feedback after a webinar lasts about a day. Send it the following morning and you get a real response set from people who still remember the middle of the session; wait until you have restructured the deck and you are asking people to reconstruct a memory, and they will answer with politeness instead.
This is also why a single well-chosen question beats a form. One question can be answered in the twenty seconds it takes to close the tab, which is the whole budget you get. Nothing here needs the platform to hold the answers for you, either โ the responses sit in your own survey tool, and your page carries the address.