I have struggled to make the suggested ways of using siri to work properly. However, I have found a way that almost always works for me:

Let's say you wan to add the task "buy milk" and it be scheduled for tomorrow. Here is the phrase use:

"Using RTM add buy milk tomorrow"

Please note however that if you say

"Using RTM add buy milk tomorrow 1pm"

the task will be added to the reminders app. It seems that adding a time causes the task to be added to the reminders app.