It is not possible to set the webhook in your account yourself. If you would like to set a webhook, please contact our support department and have them set it for you. You can simply send us your website profile along with the webhook in question and we will adjust it for you.
You can't set your own webhook, because it is an outdated feature. Nearly all modules use an automated webhook these days: the module itself sends along the webhook when creating the payment. This way the webhook is always up-to-date and manual changes are not necessary anymore. There are just a few modules left where the webhook has to be set manually.
We noticed that the webhook entry field caused more confusion than it was actually used. Which is why we decided not no show it in the dashboard anymore. Mollie staff can still fill out the webhook.