To authenticate against Bitbucket and clone repositories using GitHub Desktop, you can follow these 3 steps:

Step 1: Enable Git Credential Manager in GitHub Desktop

1. Open GitHub Desktop.
2. Go to File > Options > Advanced (on Windows) or GitHub Desktop > Preferences > Advanced (on macOS).
3. Select the checkbox for Use Git Credential Manager.

Step 2: Create a Personal Access Token in Bitbucket

1. Log in to your Bitbucket account.
2. Go to Personal Bitbucket settings in the settings dropdown.
3. Select App passwords (the side menu may be collapsed).
4. Click Create app password.
5. Under the Details section, enter a label for your password.
6. Under Permissions, select Read and Write in the Repositories section.
7. Click Create to generate a new token.
8. Copy the token to your clipboard. NOTE: You will not be able to view this again later.

Step 3: Clone Your Bitbucket Repository in GitHub Desktop

1. Open GitHub Desktop.
2. Go to File > Clone Repository > URL.
3. Enter the HTTPS clone URL of your Bitbucket repository.
4. When prompted for authentication, enter your Bitbucket username and paste the token you copied as your password.
5. Click Save and Retry to successfully clone the repository to your local machine.

This setup should allow you to authenticate against Bitbucket and clone repositories using GitHub Desktop.