I tried searching for an answer to this question but I couldn't find one. For a pull request, if I make the "Create a merge commit" and "Squash and Merge" options available, is it possible to make "Squash and Merge" the default for any PRs opened for that repository? Currently it looks like "Create a merge commit" will be the default if both options are available, and I don't see a way to change that.
Currently, there doesn't appear to be a way to change that, no. Either more than one option is available and the last one that you used is the one that is selected or there is only one option and you don't get a choice.
It looks like https://help.github.com/en/articles/configuring-commit-squashing-for-pull-requests now supports disabling the merge commit option.