If you need blank horizontal space in Ulysses, try using a UTF-8 space character.

Ulysses does not strip blank space where that blank space is a UTF-8 character, like U+2004 (three em space). While this symbol cannot be generated in Ulysses–like s̵t̵r̵i̵k̵e̵t̵h̵r̵o̵u̵g̵h̵ for example–if generated in an external program and copied to Ulysses, it will both be displayed in Ulysses and upon export.