I've created a custom pull request template (following GitHub Docs) in a subfolder of the .github directory, e.g.:
├── PULL_REQUEST_TEMPLATE
│ └── custom-template.md
└── PULL_REQUEST_TEMPLATE.md
About automation for issues and pull requests with query parameters indicates you can pass the custom template name as a query parameter when creating the pull request, e.g.
You can use the template query parameter to specify a template to automatically fill the issue or pull request body.
I've tried adding "template": "custom-template.md" as a query string when creating a new pull request but I end up with an empty pull request template. The default template in the .github directory still works as expected, but I can't figure out how to get the custom template to load via The API.