Like that.^ No jailbreak required.
Enter the Notes app and start a new Note.
Press Return as many times as you want. I made 4 lines
Double tap on the blue line and tap 'Select All' at the menu.
Then tap 'Copy'
Now back to Snapchat.
Take a picture and tap to start a caption as usual.
Tap the grey bar and tap 'Paste' when it appears.
- iOS' Notes app