That’s showing up as Header2 (## at the start of a line) in my reply window. You can also do Header3/4/5/6 as well (and Header1), but I’ve read one forum’s guide that said you have to respect the order. So the topic title is always H1, and you’re supposed to work your way through them without skipping any (though I may have already done this before I read it…)
I don’t know how strict that is though? It was quite a random find. 
Edited to add examples:
## Header2
### Header3
#### Header4
##### Header5
###### Header6