One limitation of the Authorize.net API is that you can’t have a trial length period differ from the main subscription period length. For example, you can’t have an annual membership with a 30 day trial. This is a pretty common need though.
To get around this, you can ignore Authorize.net’s trial settings and setup an annual subscription that begins 30 days out. To do this with Paid Memberships Pro, add the following code to your functions.php. This assumes that you have a level with ID 1 setup as an annual recurring subscription with an initial payment set to $0 and no trial set.
Make sure to edit the level id check to fit your site. And you can adjust the second function to output a cost description that matches your setup.