I would like to call a userevent beforesubmit in a client script. So submit is pressed in a client script and then want to forward some values that I have fetched in the client script to the user event script (beforesubmit), which must then update some records with the given values. So the that the concerned records are updated via the userevent script and not via the client script. Does anyone have a template for a client script that sends data to a userevent script?

The userevent script will fire when the record is saved. What are you actually trying to do? Logic wise, and why, not what you think the solution is

Interested in the reasoning behind the sequencing here as well; I tend to treat my entry points as stateless and independent of each other, but would need more context to give advice.

Use a Suitelet.

just set a field on the record and get it from the before submit