If I understand correctly:
As you write you want the "tab" character to write a "tab" in the text field?
If that's what you're referring to, it's standard on the web to send focus to the next UI element (button or other) on pressing tab. The only sites that capture tab presses are sites which are oriented around allowing people to write code. In most other cases, it's not desirable and actually not recommended.
TL;DR: it would be a feature to be implemented upstream by Discourse, not by us, and I doubt they will