Welcome to the Community Forum @anon10931247!
It’s currently not possible to add custom fields when you create the task emailing to Asana. you can upvote for this feature here: Parse emails to populate custom fields - #3 by Jeremy_Levy
As a workaround, I’d recommend you using our Forms feature and when you submit your form and select all your fields, the tasks will be created including the custom fields.
I hope this helps, have a nice day! 