I wonder if the OP wants to write a music cue starting at a given point in the movie without bothering too much about synchronizing the end if the cue until after it is finished.
In that case, use a separate flow for each music cue, and use the flow attachment position and/or timecode offset properties to line up ONE point in the music and video.
See How to Score to Video in Dorico | Getting Started with Dorico 2 - YouTube