Hi @Laura1 ,
Check out this tutorial, which also has example files... you can follow that same format and usage to use q2-cutadapt for demultiplexing.
I am not totally sure that that is the right tool for the job, though, if you are using this to split out hypervariable regions independently of demultiplexing. The q2-quality-control plugin also has a "filter-reads" action that can be used to filter based on alignment to a sequence (including a primer sequence), so that might be a better choice in your case.
Good luck!