Here's a handy step-by-step instruction:

1) Open the Xbox App on PC and make sure you are signed in with your Microsoft account tied to your Game Pass membership. Then click on Diablo IV and then the Install button.

2) Click “Install Battle.net” when prompted and complete the installation. (If you already have Battle.net installed, you’ll skip to the next step automatically)

3) You’ll be prompted to connect your Battle.net account to your Microsoft account. If you already have a Battle.net account you can log in to complete the connection, or from here you can create a Battle.net account to make the connection.

4) Once you have downloaded Battle.net and connected your accounts, you can complete the installation of Diablo IV!