Change config values in appsettings.json GitHubActions

I am struggling to replace values in my appsettings.json file and was wondering where I'm going wrong?

I have created two secrets in GitHub entitled COSMOSDBSETTINGS_SERVICEENDPOINT and COSMOSDBSETTINGS_AUTHKEY.

      - name: Publish
        run: dotnet publish -c Release -o ${{env.DOTNET_ROOT}}/myapp

      - name: App Settings Variable Substitution
        uses: microsoft/variable-substitution@v1
        with:
          files: '${{env.DOTNET_ROOT}}\myapp\appsettings.json'
        env:
          CosmosDbSettings:ServiceEndpoint: ${{ secrets.COSMOSDBSETTINGS_SERVICEENDPOINT }}
          CosmosDbSettings:AuthKey: ${{ secrets.COSMOSDBSETTINGS_AUTHKEY }}

I get the error stating

Run microsoft/variable-substitution@v1
Applying variable substitution on JSON file: C:\Users\runneradmin\AppData\Local\Microsoft\dotnet\myapp\appsettings.json
Skipped updating file: C:\Users\runneradmin\AppData\Local\Microsoft\dotnet\myapp\appsettings.json
Error: Error: Failed to apply variable substitution

This is the appsettings locally:

  "CosmosDbSettings": {
    "ServiceEndpoint": "https://localhost:8081/",
    "AuthKey": "KEY"
  },

Use a dot instead of a colon:

env:
  CosmosDbSettings.ServiceEndpoint: ${{ secrets.COSMOSDBSETTINGS_SERVICEENDPOINT }}
  CosmosDbSettings.AuthKey: ${{ secrets.COSMOSDBSETTINGS_AUTHKEY }}