Issue with map file selection of backend using variable

I am getting the following error when trying to use a map file and a variable to select the backend the correct backend.
“Jun 17 15:34:28 haproxy: [ALERT] 167/153428 (8967) : parsing [/local/idanywhere/etc/haproxy/conf/haproxy.cfg:531] : unknown keyword ‘use_backend %[req.artifact,map(/local/idanywhere/etc/haproxy/data/]’ in ‘frontend’ section”

I do a fetch and store it in variable req.artifact and then I use the command below to select the correct entry in the map file based on the contents of the variable. However, I cannot get past the issue with I am assuming syntax as HAProxy will not run. I have verified that the map file is in fact in the correct directory on the HAProxy server. What am I doing wrong?

req.artifact is not a know HAProxy keyword.
You might want to use %[var(req.artifact),map....