If you want to populate a custom field on your donation page with a parameter that is passed through the URL, here is how...
Select the item in Features > Sell & Fundraise > Items
Setup your custom field on the item.
Grab the custom field ID in the bottom corner when editing the custom field on an item.
4. You can then pass in a value for the field (what you want to be populated in the custom field) by adding the ?cf prefix and the custom field ID to the URL, e.g. https://myorg.com/annualevent?cf292=11/12/2020
NB: When using Webhooks, add a "Hidden" custom field that's populated from the donation page URL to pass an external reference/user ID that will get included in the payload for the webhook.