Are you referring to showing 1 specific ad in front of 1 specific video? I don't believe this can be done on a Direct Publisher channel. Generally speaking, you would enter the Ad Server url into the submission process, however with that the ads show across videos & content as a whole, and I don't believe there's a way to target it further outside of on the server side (geotargeting etc)
Gotcha. I've never done it. Considered it. I logged in tho and for CPI there is the field for Deeplink. At the top above Order Information there is an FAQ and in it it says "Deep links will begin with contentid=."?
My thinking, is it might not work for DP channels, as most of the examples seem to indicate SDK. However it's pretty thorough & I haven't had time to read it yet, nor test the deep linker url. I did CTRL+F for 'Direct Publisher' and it had zero results. If you go thru it and can't seem to get it to go on the Order Setup screen to the left says 'Need Help? Email Support' you could try that.