I'm using the mongodb helm chart and the mongo-express one. mongodb generates the name depending on my release name, so it is dynamic. The mongodb service name will be something like my-release-mongodb.
mongo-express requires to pass mongodbServer - the location at which the mongodb can be reached. How can I provide this value to mongo-express if it is generated and can change depending on the release name?
Helm doesn't directly have this ability. (See also helm - programmatically override subchart values.yaml.) It has a couple of ways to propagate configured values from a subchart to a parent but not to use computed values, or to send these values to a sibling chart.
In the particular case of Services created by a subchart, I've generally considered the Service name as part of the chart's "API": you know the Service will be named {{ .Release.Name }}-mongodb and you just have to hard-code that in the consuming chart.
If you're launching this under a single "umbrella" chart, this is a little more straightforward. Both parts have the same release name, so you can construct the service name the same way. (Umbrella charts have other limitations – if you have multiple services that each should have an independent MongoDB installation, Helm will only deploy the database once for the whole umbrella chart – but you can still hit this same problem making HTTP calls between microservices.)
If they're totally separate installations, you may need to pick the release name yourself and pass it in as a value.
helm install thedb ./mongodb
helm install theapp ./mongo-express --set serviceName=thedb-mongodb
This also a place where a still higher-level tool like Helmfile or Helmsman can come in handy, since that would let you specify these parameters in a fixed file.