jvstratum thanks for your help.
I tried:
https://www.myurl.com/index.php?option=com_acym&ctrl=frontusers&task=subscribe&hiddenlists=1&user[email]={data:mealplan_email}
added that to the curl action on the next form page after the submit button. The submit button fires and email action I have there but doesn't seem to add the curl action.
The "Use the POST command to send the data to the remote server" was already selected so i left that alone.
The "Include header in the response ?" was not selected and I left that alone.
Other than that I didn't do anything further to the cURL action.
In the Acymailing config I set:
Allow non-logged in users -to yes
Is there still something else I need to do?