One Does Not Simply

I mean how do you post a demotivator on the stream, not submit for featuring.

I don't know how to generate Demotivationals directly in a memestream, but you could make the Demotivational first, then either:

a) download the image, then upload it to the stream and make it as a meme

b) copy the link, and put a meme in the stream saying "For my demotivational suggestion, click here" (or similar), and put the link to it in the comments