How to make custom track key in stick-table

Can i make a custom track key from map file below
I want to change key by unix socket cli

if Anyone has ideas let me know !

http-request set-var-fmt(txn.custom_key) str(custom_key),map(/etc/haproxy/etc/
http-request track-sc0 var(txn.custom_key) table xxx if xxx

http-request set-var(txn.custom) req.hdr(%[var(costom_key)])

custom_key example