Thanks @Alvin for pointing me to the right direction. However, the scheduler in his yml file is somehow not recognized by Azure, even though the file is validated with no issue. I had to modify it a bit, not sure what is wrong.

Here is my modified yml:

# ASP.NET Core

# Build and test ASP.NET Core projects targeting .NET Core.
# Add steps that run tests, create a NuGet package, deploy, and more:
# https://docs.microsoft.com/azure/devops/pipelines/languages/dotnet-core
#inputs options: https://github.com/tinglesoftware/dependabot-azure-devops/blob/main/extension/README.md
trigger: none 
schedules:
- cron: 0 13 * * 0 # 1pm in UTC = 6/7am PST cron syntax defining a schedule
  displayName: weekly build on Sunday # friendly name given to a specific schedule
  branches:
    include: [ dev ] # which branches the schedule applies to
  always: true # whether to always run the pipeline at the scheduled time, even if there have been no changes
  batch: false # whether to run the pipeline if the previously scheduled run is in-progress; the default is false
jobs:
  - job: null
    displayName: Scan dependencies
    pool:
      name: Azure Pipelines
      vmImage: ubuntu-latest    
    steps:    
    - task: dependabot@2
      displayName: 'Run Dependabot'
      inputs:
        autoApprove: true
        setAutoComplete: true