To get the connection string, just open the Checks tab in your Pull Request and scroll to the bottom. There you can connect either directly per SSH or via a web based terminal.

YML

I have a simple YML action workflow to try this action out

name: CI

on: [push]

jobs:
  build:

    runs-on:  ubuntu-latest
    
    steps:
    - uses: actions/checkout@v1
    
    - name: Setup tmate session
      uses: mxschmitt/action-tmate@v1

Screenshots

I have looked on the PR itself and can not find it and the streaming logs is not listing anything out for me to see the URL, only when I cancel/stop the run can I see the log output and what the tmate.io URL was.