The command string you say that appears as a single space is appearing as 8 spaces here, not a tab (but maybe that's StackExchange).
Debugging this may be a bit tricky. There are several things that may or may not "cook" tabs: the terminal, when inserting the character; the shell, when processing the input; the underlying terminal mode (cooked mode vs. raw mode), and, in the end, dialog itself.
To make things even more confusing, if you use echo to generate a tab, or if you somehow else generate a tab, there's still a chance the terminal emulator decides to wash that off.
A commonly seen keystroke to escape this kind of control characters is
C-v (that is,
CTRL+V). If you hit that, then tab, you may get a real tab inserted. Here with the GNU
urxvt, it works.
Properly debugging this requires knowing exactly what's the issue (that is, what's eating the tab). Simply putting \t there won't work unless either
dialog or your shell have the feature to translate these escaped sequences. If you want those, then follow @Eran Ben-Natan's advice and use GNU
echo's enhanced mode (other implementations may/will differ — there's no specific behavior specified in POSIX (IEEE 1003.1 2008, page 2615, line 84306)).