You can automate with the Actions panel.
Open an image you want to adjust -- you must have an image open in order to create an action.
In the Actions panel, click the New Action icon at the bottom of the panel.
Give the action a name and click Record.
Make the Levels adjustment that you want and click OK.
If that's all you want in the Action, click the Stop/Playing Recording button at the bottom of the Actions panel.
You can now close your file and choose whether or not to save it.
Since you are applying the Action to multiple files, you can set up a Batch action. Before doing that, it's best to put all the images to be processed into a folder, and create another folder for the processed files to be saved into.
Under the File menu, choose Automate > Batch.
Choose the action, the folder of the source files, the folder for the processed files, and click OK.
Make the Levels adjustment.
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