Altium Designer - How to make chamfer on 90 degrees tracks?
Personally I don't add chamfers manually. I use the "Teardrop" feature (found under Tools > Teardrops, or shortcut T-E):




Using the above settings, this is what I get on a T-junction:


It also works on pads (through-hole AND SMD):


Using teardrops really helps improve the look of your layout, and avoids any sharp angles that could cause issues during manufacturing. In the Teardrops dialog, in the Options pane, you could set the "Teardrop style" to "Line" instead of "Curved" to get the exact chamfer you ask about:





EDIT: This is based on the assumption that you are running Altium version 14.3 or higher. Before that, the Teardrops menu was different and did not have support for T-junction teardrops. In that case, instead of using a regular T-junction, I would recommend a connection like this:



If you really want a filled chamfer, you can draw the above with the traces, then place a triangular solid region in the empty area:



This ends up looking like this: