Before LAMBDA was introduced to Excel, you'd need to know how to use VBA or macros to create your own function. However, today, Excel's LAMBDA tool lets you do this using familiar Excel language ...

Press F10, and small letters appear over each ribbon tab and quick access toolbar item. Alt+H takes you to the Home tab, ...

There are two other methods to use the BESSELI function Method one is to click the fx button on the top left of the Excel worksheet. An Insert Function dialog box will appear.