Here are the triggers you can choose from:

Trigger by Data Capture = When someone texts a keyword, they go through the data capture process, and then are automatically added to the AMC. People who text in the keyword will be added to the relevant group, data capture, and your AMC. *This is the trigger we recommend using in most cases

Trigger by Group = Any time you add someone NEW to the group selected, they will also be added to the AMC. *Please note if there are contacts in the group BEFORE you select this option, they will NOT be added to the AMC. Only NEW contacts added to the group after this trigger is set up will be added to the AMC.

Trigger by Keyword (NOT recommended) = When someone texts a keyword, they would be put directly into the AMC.
​NOTE: Currently, we don't recommend using the keyword associated directly with the AMC. You can use keywords related to groups and/or a Data Capture (DC), and then have that linked to the AMC. Read more here.

On the Campaigns page when you choose to add a new AMC, the Create an Automated Messages Campaign box will pop up. 

After you choose a name and click Next (brown box above), you will get the Automated Messages page for adding a new AMC. 

Click on Edit Triggers (pencil icon, brown box above) and the Edit Triggers box will pop up. 
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Brown box above: Toggle the button on (it will turn purple as in the screenshot below). Click on the SELECT DATA CAPTURE(S) line. A dropdown list will open up. Select the data capture you wish to link to this AMC.

Blue box above: Click anywhere on the SELECT GROUP line and choose from the dropdown list.

Yellow squiggle: Not recommended.

Choose your trigger(s) and click Save. 

Read more here.