To automatically set the assignee on an issue using GitHub Actions, you can use the assign action in a workflow file. Here is an example of how to do this:

name: Set assignee on issue
on:
  issues:
    types: [opened]

jobs:
  set-assignee:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - name: Set assignee
      uses: bahmutov/assign-action@v1
      with:
        assignees: 'username1,username2'

In this example, the assign action will be triggered whenever a new issue is opened. The action will then set the specified assignees (in this case, username1 and username2) as the assignees for the issue.

You can also use the assign action to set the assignee based on other conditions, such as the label on the issue or the value of a specific field in the issue. For more information and examples, you can check out the assign action documentation on GitHub.