If you don't want to use content control data binding, as I previously recommended in your other post...
You'll need to find the text box first. To do that, you can use XPath or TraversalUtil.
To find it, you'll need to know what object represents a textbox. To find that out, upload your docx to the webapp (linked in the menu above)
After that, it is just like adding an image (as to which see the ImageAdd sample)
(Or instead of the above approach, you could do what VariableReplace does, which is a JAXB marshal/unmarshal cycle, but again, you're on your own with that method)