It's a general practice to access application menus by holding the Alt key and then pressing the character defined as its access key. For example, to open the File menu of any Windows application, we use Alt + F, and to access the File | New menu, we use Alt + F, N. Here, the character F and N are used as access keys that are invoked when we press Alt.
In the WPF application, you need to specify _ (underscore) before the character you want to highlight as the access key. For example, adding _ before the F in File menu header content activates the said menu when Alt + F is pressed:
<MenuItem Header="_File">
<MenuItem Header="_New" />
<MenuItem Header="_Open" />
</MenuItem>
The frequently used practice is to use the first character that's not already used as an access key of another control. But, on a need basis, you can specify any character part of the label content.