The CURP paper doesn't define a new leader election algorithm and assumes you use an existing one like raft¹. Xline is using raft based on the readme

https://github.com/xline-kv/Xline/tree/master/curp/tla%2B

¹warning, I skimmed and ctrl-fd for "leader election"

For those interested in this sort of thing, see also Raft's TLA+ specification: https://github.com/ongardie/raft.tla