Unable to change source for Media Texture

I’m trying to place a Media Player inside of a UMG Widget on the Image aspect. That’s not the problem, even if that won’t work.

When I create a media player and include a video asset, it creates the Media Player and corresponding Media Texture file, but the Media texture file has ‘No Path Set’ under source, and the “…” to select a source is locked.
When I try and create a Media Texture file on it’s own, the same thing happens.

If I force the video into the Media Player Editor, then I can see how it should look, but the screen is black when the game is played and the source is just black. (The Media Player Editor is apparently more for testing, and some of the changes do not stay).

I am using 4.17, and I cannot move up to 4.18 because of a plugin that I need.
The videos are MP4s

In 4.17, you have to point the MediaPlayer’s VideoTexture to the MediaTexture asset that will receive the video output.

You can also set this property from Blueprint via the MediaPlayer’s SetVideoTexture function.