There appears to be no way of specifying the initial attachment name when creating an addon (--as parameter on the Heroku command line, also appears to be referred to in the docs as an alias).

This is exposed fairly nicely in the Heroku Go client, https://github.com/heroku/heroku-go/blob/daded33ff8cdcd8127dcfc3c744686ae2e8d4efc/v5/heroku.go#L388-L390

It doesn't seem that far off in it's current state, as it is constructing this very same object, but providing only Name, Config and Plan (everything but Attachment!): https://github.com/terraform-providers/terraform-provider-heroku/blob/4f59dde/heroku/resource_heroku_addon.go#L99