To start a workflow manually it must have a trigger called workflow_dispatch:

---
name: Tests
on:
  push:
    branches:
    - master
  pull_request:
    branches:
    - master
  workflow_dispatch:
    branches:
    - master  

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In the Actions tab of your repo you can now select a workflow and press "Run Workflow".

See GitHub documentation for details Show archive.org snapshot .