Tcl add a symbol N time at the end of the string
% format "|%-25s|" hello
|hello                    |

% set width 25
25
% format "|%-*s|" $width hello
|hello                    |

set padded [format "%-25s" $str]

set padded $str[string repeat " " [expr {25 - [string length $str]}]]

for {set padded $str} {[string length $padded] < 25} {} {
    append padded " "
}

set padded [binary format "A25" $str]

% string repeat : 25
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