Hello hibatt,

1. For e-file, ensure that you've selected the TX extension on the Profile tab

2. Proceed to General > Automatic Extension (7004) > State & Local (on the top menu bar)

3. Go to Texas Extension Request and ensure you've enter a 1 in the "1=automatic extension, 2=blank (MANDATORY)" field. 

The extension payment will calculate automatically, however you can use the override for "Tentative tax (-1=none) [Override]" in order to manually choose the extension payment.