Spotify is a proprietary and not open source music streaming player: a freeware.

As such it is not included in Slint.

However you can build then install it this way:
lftp -c "mirror https://slackware.uk/slint/x86_64/slint-15.0/source/spotify"
fakeroot sh spotify.SlackBuild
sudo installpkg *txz

To run it, if not done you will need to install the package libcurl-gnutls, available in Slint,
typing as root or using sudo:
slapt-get -u
then either:
slapt-get -i libcurl-gnutls
or:
slapt-get --upgrade