Is it possible to pass the username in a link to site so that the user does not have to key it in, and just provide the password ?
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Yes, https://site.appiancloud.com/suite/?un=user.name
Great, works!
Is it possible to pass further parameters to be used inside ?
My use case is a link which is being sent via email to a user with some task id.
The desired functionality is to click the link (in the mail), enter the password and
then redirect to the form activated by the associated task - possible ?
Yes, if that email is sent from Appian you can use the function a!urlForTask() to get the task URL. If you append '?un=user.name' to the end of the URL it should continue to work.