Is there any digital watermark technology for raw text? I have a demand to protect my data which can accessed by my client privately.
My data is not in any document but in plain text, e.g., a JSON string.
I want to protect my data against being modified, then redistributing without my permission.

I researched online for a while, and concluded that,


there are digital watermark methods for protecting images, audio, videos or documents (pdf, MS Word doc or other format),
but there is no such methods for raw text. 


I want to know if there is any cryptographic way to achieve my goal?
I assume the request for clarification of "Plain-Text" has to do with content, not that it's an ASCII or UTF string.

There is not a "Hello World!" string that I could make that you couldn't then copy and call your own, where I could 100% prove that it was mine to begin with. For example anyone can just delete a PGP signature wrapper.

However, if the content of your string is structured in a way that was uniquely identifiably yours then I would call that a watermark, as any change to the content of string would change its context. For example using a private RSA key to encrypt your string then coding it back into ASCII with Ascii85, or if your string contains obfuscated JavaScript, or even simply something you've published under a copy right

That should afford about the same level of watermark you'd see in the other media, but the thing is any media which can be displayed with out the watermark is by the reflexive property media that can be stripped of its watermark and not still remain self judicating. ie copy right it, or have context that's destroyed when changed.