After login navigate to Trigger.
There on the right hand side you will find option to add trigger, click on it.
A new window will open where you will be asked to select event(select missed call) and press next arrow.
On the next screen, select event type(select sms) and press next arrow.
Select to whom do you want to send auto sms(select caller)
Enter the desired sms content to be sent as an auto-reply, and click preview.
In case you need to add some variables in the auto reply sms, click on the appropriate links to insert the variables in sms content. Click on preview button.
On the next screen verify the details and click on submit.