I’d like to type out a Draft and then enter a Things to-do from the bottom of the Draft. However, the Send to Things syntax pulls the entire draft in. Is there a way to change the syntax so that Send to Things only pulls one line (maybe with a - in front of the todo)?
things:add?title=[[line|2]]
This would use the second line as the title. See more possibilities here: Drafts Templates | Drafts User Guide
@CMx3 is point you in the right direction.
If you just want to alter the existing action to send different text, you would alter the URL template with different template tags. In your case the most useful possible options are [[selection]] which sends only the selected text, or [[line]] which sends the current line the cursor is on (saves needing to make a selection).
You might also look at more advanced examples that can work off markup in your text, like the Things Parser.
Thanks @agiletortoise! That definitely gives me some options.