It is actually impossible (or I didn't find how to) use the keyboard to drive xfce4-popup-applicationsmenu. I have binded this command with the key combination CTRL+SUPER (to have a behavior close to the windows one) but then, it's only possible to navigate into this with directional arrows. I could be very nice to be able to access elements by typing their first letter. And if there are more than one element with the same first letter, navigate into them.* By example, typing CTRL+SUPER to call the menu, then 'd' to go in development section, and finally 'c' for codeblocks. Another interesting thing, if you make this idea real, should be to underline the shortcut letter. *: Or use a different letter, like the 2nd, but this is not really interesting. Maybe give choice in the configuration...
As whiskermenu covers this use-case I don't think we will be implementing this feature anytime soon. Closing as wontfix for now.