Hey there,
I am sorry for the delay of this reply,
I assume you would like to pass subscription start date and expired date right?
If yes, you can check my following details and my attached screenshot for further explanation, hope this will help.
subscription_date=[[SUBSCRIPTION_SUBSCRIPTION_DATE]]
expiration_date=[[SUBSCRIPTION_EXPIRATION_DATE]]
Currently we do not have these documentation yet, if you would like to add more query variable you can check on that "view token" section and your site database table, these key `subscription_date` is follow from this table #__payplans_subscription if you would like to send subscription start date.