1. Click the "Home" tab in Power BI and then click the "Edit Queries" button.

2. Then, under "Queries" on the left side of the screen, click on the name of the table you want to add the index to.

3.Then click on the "Add Column" tab and then click the "Index Column" button. (If you click the down arrow beside the Index Column button, you can decide what number to start the index with.)