Now your Rule has been created then you can specify what Actions are to take place when the Rule is triggered.  In the Action tab press [New] to launch the Action pop up.

Select Type - this will display all the current actions available, select 'Set custom Security Groups field' and the field to the right will update its label based on the Type you have selected.

Fig 1 - Custom Security Group  selection

Select the Custom field required using the drop down list.

In the Security Groups field - select the Security Groups that should be defaulted 

Once all the Security Groups are selected press [Save] the Action will now be added to you Rule.

Multiple Actions

If you wish to trigger multiple Actions for your Rule, click the new button again and add a new Action.  You can add as many different Actions as you wish. 

For more information regarding the Custom Security Group field, please read this article.