Getting id after inserting row(Spring boot/JPA)

I have trouble with getting id from one table after a new user is added. I have 2 tables, the user and the other one that has to be joined with the user table into the third table which will contain id from those 2 tables. For that, I need the id of the second table, which if I understood correctly have to be collected after inserting row. So my question is how to implement that getting id? So if anyone could help, I would really appreciate it.