Getting a screen capture depends on the platform / device being used to access CC. The end result will usually be a file containing the image. That file can be uploaded to a CC post.
On some platforms the image can be dragged and dropped in the edit box. Even cut and pasted.
Independent of platform, at the bottom of the edit box there is an Add Files link. Click / tap the link, select the screen capture file. Once it uploads, place the edit cursor where you want the screen shot placed in the post and click / tap the Insert link.