Formating issue: default value for undefined variable

You need to make sure you use the expr syntax when using a ternary operator within a variable.

For example:

% set colour [expr {"1" ? "red" : "blue"}]
red
% set colour [expr {"0" ? "red" : "blue"}]
blue

So looking at your example, you'd need to change it to be something like this.. (note that I have changed the from single to double quotation marks for the hyphen)

set message  [format \
    "%s - %s \[%s\]..." \
    ... \
    [expr {[info exists [HTTP::username]] ? [HTTP::username] : "-" }]
]