Lookup value from map using literal key


We’re trying to setup haproxy to fetch a specific backend from a map, but currently the best way we’ve found to set the key we want to lookup in the map is by setting a custom header then looking up that custom header value in the map, as part of the use_backend.

use_backend %[req.hdr(X-Custom-Header),lower,map(not_host_map.map)]```

We're wondering if there is a way to specify a literal lookup into a map?