Just found it, in GUI Editor. Messed around it and got it worked. Like the name says: "Gui
AviBitmapCtrl" It only accepts AVI files nothing else.
Here my successful screenshot:

It could be useful for users that wanna force people to download their Client_BLAH.zip Just include the AVI file and if you enter the Add-On Creator's Server then the AVI File will be played(Intro will be played) then the AVI GUI disappears and Game begins.
I've just had the feeling to share it, because it wasn't discussed here before, or?
/discuss
Tutorial - How to add GuiAviBitmapCtrl and getting it working?Requirements:
The Mission Editor(If you don't know how to set up, forum search helps.)
Blockland.exe(I think everyone has that.)
HyperCam 2(or another video software that records videos in AVI)
Let's start! After your blockland started, press F10 and the GUI Editor opens. Now go to File and press New Gui... You see that GuiControl Box. Doesn't change anything in settings. Press Ok. So now you have a empty GUI. Now go to New Control and search for: "GuiAviBitmapCtrl" if you found it, put it somewhere on the field. Now record your own video with HyperCam 2(or another video software that records AVI videos). After you did, go and save your video somewhere in the Blockland Folder. I used base/client/ui/myclip.avi for example, but it's your choice where you wanna put it in(But it have to be somewhere in the Blockland Folder!).
Good. After you saved it, go to GUI Editor again and press on GuiAviBitmapCtrl, look at your right, you see a list with settings. Now search for "aviFilename" if you found it, then put your video filepath in there. Now go up and press Apply. It should lag for a second, but that isn't bad either. Just wait a minute, and then your video runs!
If you do not want using that way, and you are familiar with TorqueScript then you can also use this:
//--- OBJECT WRITE BEGIN ---
new GuiControl(NewGui) {
profile = "GuiDefaultProfile";
horizSizing = "right";
vertSizing = "bottom";
position = "0 0";
extent = "1024 768";
minExtent = "8 2";
visible = "1";
new GuiAviBitmapCtrl() {
profile = "GuiDefaultProfile";
horizSizing = "relative";
vertSizing = "relative";
position = "6 1";
extent = "1013 761";
minExtent = "8 2";
visible = "1";
aviFilename = "base/client/ui/yourclip.avi";
swapRB = "1";
done = "1";
letterBox = "0";
speed = "1";
};
};
//--- OBJECT WRITE END ---