As you don't want to expose the data, so it's better to have it saved as secret in kubernetes.

First of all, add this two lines in your Values file, so that these two values can be set from outside.

username: root
password: password
Now, add a secret.yaml file inside your template folder. and, copy this code snippet into that file.

apiVersion: v1
kind: Secret
metadata:
  name: {{ .Release.Name }}-auth
data:
  password: {{ .Values.password | b64enc }}
  username: {{ .Values.username | b64enc }}
Now tweak your deployment yaml template and make changes in env section, like this

...
...
    spec:
      restartPolicy: Always
      containers:
        - name: sample-app
          image: "sample-app:latest"
          imagePullPolicy: Always
          env:          
          - name: "USERNAME"
            valueFrom:
              secretKeyRef:
                key:  username
                name: {{ .Release.Name }}-auth
          - name: "PASSWORD"
            valueFrom:
              secretKeyRef:
                key:  password
                name: {{ .Release.Name }}-auth
...
...

If you have modified your template correctly for --set flag, you can set this using environment variable.

$ export USERNAME=root-user
Now use this variable while running helm install,

$ helm install --set username=$USERNAME ./mychart
If you run this helm install in dry-run mode, you can verify the changes,

$ helm install --dry-run --set username=$USERNAME --debug ./mychart