As other member said, spoiler tag isn't exist. But looking at your signature, i would suggest you to put NSFW warning (with bigger font size, bold or red/blue color) at top of your thread instead.
I remember seeing someone with a live cam porn signature in the past. so I thought having NSFW content without warning on the signature is allowed and an NSFW warning will only be needed to be put if the NSFW content is on an avatar, on a post, or a thread.
I know NSFW content is allowed here, but i assumed OP don't want other member see it accidentally (especially on public space or office).
that being said, the OP's thread about the live cam site should have an NSFW on the Title.
The thread already have [18+] on the title though.
Been suggested way back 2012. Guess what? Even the Global Mod @hilariousandco already made a suggestion regarding the feature. I feel the way as well to have this spoiler tag but I don't think if this will be implemented here maybe on the developing epochtalk forum.
I just had a look at smf forum and searched for spoiler tag and found this thread-
Inline Hover Spoiler BBCode. This is created back in 2008 and updated in 2014 but sadly I can't see 1.1.19 (bitcointalk smf version) in compatible list. Maybe that's the reason?
Total line of the source code is only 113 lines (29, 37 and 47 from 3 different files), surely someone could modify it without lots of effort to make it compatible bitcointalk SMF.