How to access a secret injected into a Kubernetes pod with Vault?

Question

I am new to using Kubernetes. I have a Kubernetes cluster with 8 microservices, and all services are deployed using Helm charts. While I specify some parameters in values.yaml, such as database passwords, I don't want to store sensitive information like passwords in values.yaml. I have injected a test secret into my services' pods using HashiCorp Vault, and I can access this secret at /vault/secrets/test path. My question is whether I should access these secrets from within my C# code or if I need to specify something in my testapplication.yaml?

Here is how I access my DB_Password:

 spec:
      containers:
        - env:
            - name: DB_Password
              value: {{ .Values.mssqlLoginPassword }}

I inject secret with this way on testapplication.yaml for test:

spec:
  template:
    metadata:
      annotations:
        vault.hashicorp.com/agent-inject: "true"
        vault.hashicorp.com/tls-skip-verify: "true"
        vault.hashicorp.com/agent-inject-secret-test: "secret/prod-secret/test"
        vault.hashicorp.com/agent-inject-template-test: |
          {{`{{- with secret "secret/prod-secret/test" -}}
          {
            "username" : "{{ .Data.username }}"
          }
          {{- end }}`}}
        vault.hashicorp.com/role: "prod-secret-role"

...

I tried these solutions but didn't work for me;

1. Changing annotation

      annotations:
        vault.hashicorp.com/agent-inject: "true"
        vault.hashicorp.com/tls-skip-verify: "true"
        vault.hashicorp.com/agent-inject-secret-test: "secret/prod-secret/test"
        vault.hashicorp.com/agent-inject-template-test: |
          {{`{{- with secret "secret/prod-secret/test" -}}
          {
            export username="{{ .Data.username }}"
          }
          {{- end }}`}}
        vault.hashicorp.com/role: "prod-secret-role"

2. Add this so the application container can source those files during startup

      containers:
          args:
            ['source /vault/secrets/test && <entrypoint script>']

I am not very familiar with Kubernetes, so I would be very happy if you could explain the solution in detail.