A client church wanted an easy way to allow people to enable SMS messages when they register for an event. Since the SMS Enabled field isn't available to add to the registration form, we had to find another way to accomplish this. It also had to be easy. We didn't want to have to remember to add a registration attribute with a specific key. We didn't want to use a registration workflow or registration instance workflow since that would prohibit them from using any other workflows. This is the simple process that we came up with. 

First, create a new person attribute with a field type Boolean. You can name it whatever makes sense to you. You will need to make note of the Id of the attribute.

Next, download the Update SMS Enabled based on Person Attribute workflow using the button at the bottom of this recipe. Import the Workflow under Power Tools > Workflow Import/Export.

Next, go to Workflow Triggers and create a new Post-Save trigger as shown below.

• Make sure to put your Id in the Or Value Is field.
• Make sure to choose the Update SMS Enabled based on Person Attribute workflow in the Workflow Type field.

Now all you need to do is add your new person attribute to any registration form that you want to allow people to opt-in to SMS messages. You can also use this person attribute on any other form or page that allows editing person attributes but not the SMS Enabled field.