Hi, welcome to the forum,
Floating text is not added to an image but to a page. It's a printing function rather than an image function.
You can add some Image Info' text in the Batch Filter which would be added to your e-mail copy but it's all related to the image from exif data and the positions are fixed.

For complete freedom for text on an image you have to use the Print to File (PTF) function (File menu or Menu Cheater button). This creates a new image from the page, so you can have Borders and Floating text.
For a single image per page, specify a page size and resolution (in the PTF dialogue) to get the e-mail size you require, select the Monitor/Web option and OK. Next add your images to the queue, crop scissors down to fill the page and then add your text in the Full Page Editor. Click Print to get a new set of images with the Floating text.
Terry.