I need to make a ton of GIFs that all end with "-tn" like so:
FILE_NAME-tn.gif

Is there a way to add this text string in the save for web dialog box that I am using in my action?

Thanks,

Bill

Yes...

Go to your Save For Web dialogue.

At the top right, look for the little right-pointing triangle in the corner of the "Presets" panel.

Click it, and drop the menu. Glide down and select "Edit Output Settings..."

A modal dialogue window for SFW output preferences will open.

Drop the menu and select "Naming Files."

Here you will see a bunch of editable fields, most of which can be configured to add the file-name text appendage you desire. Explore these...it'll take less time for you to figure out than it would to explain exactly what you need to do.