Support email pre-filling for the customer portal sign-in via url parameter

[saniul1]

email pre-filling for the customer portal sign-in form is not supported in the same way as it is for checkout links. i would like to send customer to https://sandbox.polar.sh/authtest/portal/request?customerEmail=usermail@gmail.com like this and email to be autofilled. it helps so user don't have to remember what email they used to purchase.