custom css field within the text-block

Dear Support Team

I use my own css on my subtitle page, which I defined in quick css as follows:
.text-bu {font-size: 11px; line-height: 14px! important; }

If I now create a new text block, there is no field for “custom css class”. I have seen in another post that this possible sit and there appears below in the text block an input field for “custom css class” see: https://imgur.com/a/gg9ylpL

how can I make this ad visible in my text block as well?
Thank you and VG Diana