This command: ./mach try -b d -p win64 -u mochitest-o[Windows 8] -t none[Windows 8] produces the error message: The try command does not accept the arguments: 8] I got that command from try chooser. I tried dropping the platform specifier on none ./mach try -b d -p win64 -u mochitest-o[Windows 8] -t none but that produced this error: Error parsing -u argument (['mochitest-o[Windows']): Maybe this is just a bug with try chooser and spaces need to be escaped or something.

possibly related to bug 1263071 and bug 1283163

Escaping spaces "\ " makes it work I think. Example: https://treeherder.mozilla.org/#/jobs?repo=try&revision=9c8319a800590506d3d69971ec64bc5204c19c24

Right - escaping spaces is necessary if there is a space in the platform name: -u "mochitest[Windows 8]"