How to add Text if a Checkbox is ticked? See screenshot attached

It probably could be done in Acrobat somehow. Acrobat has page templates, so they perhaps could be used.

Alternatively, you could have the actual contract text as one or more big text fields. A script in Acrobat could then insert the desired text in the text field. If the formatting is fairly conservative, as appears in the screenshot, a text field could handle it.

Not so simple, especially if the whole contract doesn't fit on a single page. But probably doable if it's very important to do it that way.