Drafts works great with iOS dictation, just launch Drafts and tap the dictation button. That said, sometimes it's handy to be able to create a draft directly via Siri without ever having to unlock your phone and launch Drafts.
The trick to accomplishing this is via Drafts' "Import from Reminders" feature. You can read more about Drafts' Reminders integration, but, in short, if you enable the "Import from Reminders" switch in Drafts' settings, whenever Drafts is opened it will check for a list named "Drafts" in the iOS Reminders app and import any incomplete reminders with the title as the first line and any notes as the body of the draft. It will then mark those imported reminders complete to prevent duplicate imports.
Siri is capable of creating reminders in specific lists, so to use Siri to create a draft, simply start a Siri command with "Add Drafts reminder...", or "Add to my Drafts list..." – then dictate the remainder of the text you want in the draft. Confirm the Siri reminder when she asks, then the next time you launch drafts, you will find the imported reminders at the top of your draft list.
Give it a try, it's handy!