For the dictation mode, speaking "semicolon" should be transcribed as ";".
Are you looking to insert that symbol as part of a predefined command phrase instead?
If so, while you can't enter the character, you could replace the literal text "semicolon" with the symbol, either using the "{TXTREPLACEVAR:}" token, or the "Replace with" feature of the "Set a Text Value" action.
E.G.
Quick Input, '{TXTREPLACEVAR:"{CMD}":"semicolon":";"}'
or
Set text [~output] to '{CMD}' (Replace 'semicolon' with ';')
Set Windows clipboard to '{TXT:~output}'
Press Left Ctrl+V keys and hold for 0,01 seconds and release
(either output method should work for either replacement method)
Of course, if all your command does is insert the character, you could instead use a keypress action set to press the relevant key on your keyboard that produces said character.