Check custom fields you need on the "Download settings" tab at the PowerBI Integration Bitrix24 application. Save changes and make sure you cleared the cache using the button at the bottom of the page.
Tip: choose only those fields you really need, because each field adds to the total time of downloading data and makes the process slower.
Keep in mind that each portal has it's own custom fields therefore you need to activate those fields in your template individually after the transfering data
1. Download your data to PowerBI template and click "Transform Data"
2. Select the table you need, for example "Deals". If you are looking for a field from Lead, Contact or Company, select the "Clients" table.
3. Look for "Extended json" in Applied Steps and click the "Gear" button on it.
4. The "Expand Json" pop-up will apear, click "Load more" there.
5. Mark the custom fields you need from the list and click "Ok".
6. Click "Close & Apply".
7. The custom fields now will be shown in your report.
P.S. If you need to know the system field name, point your cursor on it at the page CRM->Settings->Form and report settings->Custom fields
Or you can open the settings of the field you need and see it's system name at the URL.