You can't seek track position using the track seek bar like it does when using Music app. For seek bar to work with your app, the track should be in the device's music library. I noticed quite a few apps which support lock screen/control center track seeking, they get their tracks from the Music Library in the device. Apps like SoundCloud don't support seeking track to position at this point of time
The only way to support forward and backward seeking is by the previous/next buttons which send a begin/end seeking notification when long pressed.
UIEventSubtypeRemoteControlBeginSeekingBackward, UIEventSubtypeRemoteControlBeginSeekingForward,
UIEventSubtypeRemoteControlEndSeekingBackward, UIEventSubtypeRemoteControlEndSeekingForward
According to the other answers it seems that seeking is not possible right now.
https://mcmap.net/q/1468214/-setting-and-accessing-ios-seek-bar-on-lock-screen, https://mcmap.net/q/672752/-how-to-enable-audio-scrubber-in-ios-lock-screen-control-panel