How can I put my local client's video ad on Roku

Since this is for limited viewing, you won't have to officially publish a "channel" (app) on Roku's channel store - instead clients would be able to add the so-called "private channel" (app) to their desired Rokus by using a so called "access code". You'll need an app developed for (or by) you - or see the announced last year "Direct Publisher" (i am unclear if that can be used for private channels) by simply writing video descriptor in json or xml

If custom app, further there could be couple of different approaches - either have 1 app, "your" app - in which clients enter login or magic code to access their videos - or if your business is fancier, each client may be handed their "own" app which once installed knows where to find the videos.

Completely separate issue left to you is where to host the videos (Roku does not host videos, it just plays them) - whether on YouTube (beware, people will tell you it goes against YT terms to use non-YouTube player app), Vimeo etc or your own servers.

Wait, that's a good idea in it's own but not what I was talking about.

I have seen local businesses' ads on Roku channels in preroll.  Spectrum produced the ads and I'm sure made a media buy on cable and also on Roku for digital media.  
Makes me think the cable companies are feeling the sting or seeing the writing on the wall.

Oh, you would like to advertise on Roku?
See https://www.roku.com/advertising - there should be contact info how to get in touch with them

See also this recent post https://blog.roku.com/developer/2016/12 ... -roku-ads/