Find the Spotify D-BUS interface (make sure Spotify is running):

dbus-send \
  --session \
  --type=method_call \
  --print-reply \
  --dest=org.freedesktop.DBus \
  /org/freedesktop/DBus org.freedesktop.DBus.ListNames

# Irrelevant output truncated.
org.mpris.MediaPlayer2.spotify

See the available methods in Spotify’s D-Bus service:

dbus-send \
  --session \
  --type=method_call \
  --print-reply \
  --dest=org.mpris.MediaPlayer2.spotify \
  /org/mpris/MediaPlayer2 org.freedesktop.DBus.Introspectable.Introspect

# Irrelevant output truncated.
<interface name="org.mpris.MediaPlayer2.Player">
    <method name="Next"/>
    <method name="Previous"/>
    <method name="Pause"/>
    <method name="PlayPause"/>
    <method name="Stop"/>
    <method name="Play"/>
    <method name="Seek">
      <arg type="x" name="Offset" direction="in"/>
    </method>
    <method name="SetPosition">
      <arg type="o" name="TrackId" direction="in"/>
      <arg type="x" name="Position" direction="in"/>
    </method>
    <method name="OpenUri">
      <arg type="s" name="Uri" direction="in"/>
    </method>
    <signal name="Seeked">
      <arg type="x" name="Position"/>
    </signal>
    <property type="s" name="PlaybackStatus" access="read"/>
    <property type="s" name="LoopStatus" access="readwrite"/>
    <property type="d" name="Rate" access="readwrite"/>
    <property type="b" name="Shuffle" access="readwrite"/>
    <property type="a{sv}" name="Metadata" access="read"/>
    <property type="d" name="Volume" access="readwrite"/>
    <property type="x" name="Position" access="read"/>
    <property type="d" name="MinimumRate" access="read"/>
    <property type="d" name="MaximumRate" access="read"/>
    <property type="b" name="CanGoNext" access="read"/>
    <property type="b" name="CanGoPrevious" access="read"/>
    <property type="b" name="CanPlay" access="read"/>
    <property type="b" name="CanPause" access="read"/>
    <property type="b" name="CanSeek" access="read"/>
    <property type="b" name="CanControl" access="read"/>
  </interface>

Use the available method as part of a script, keybind, etc.:

dbus-send \
  --session \
  --type=method_call \
  --print-reply \
  --dest=org.mpris.MediaPlayer2.spotify \
  /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.PlayPause