To play music on the OpenSwim Pro, you can transfer MP3 files directly to the device. To do this, connect the headphones to your computer using the 4-pin magnetic cable supplied. Then open the virtual drive "SWIM PRO" on your computer and drag the desired music files into it. Supported formats are MP3, FLAC, WMA, WAV, AAC, M4A and APE. Alternatively, you can also use Bluetooth to stream music from a connected device.
As I understand it, you cannot use the offline functionality of Spotify.
To be able to listen to music while swimming, you need to have audio files in .mp3 or .m4a format, for example, and copy them from the PC to the headphones as you would to a USB stick.