Transmission has a number of CLI configuration parameters (see here), one of which is download-dir. This sets the 'default' download directory.
Using the default or kettu web UIs this box is prefilled to the value of download-dir. See screenshot:

(Download directory is pre-filled)
Whereas in combustion UI this does not happen:

(Download directory is empty, displaying NaN TB free)
I think combustion should autofill this box based on the download-dir parameter.
Observing the source code, it appears this is the intended behaviour this.torrentUpload.setDownloadDir(this.props.session_store.settings['download-dir']); in index.js, however for some reason this does not happen.