I get the impression that people think that linking to emails from Agenda is only possible on Macs or iPads that allow split screen displays of Agenda and mail
Turns out it is possible on all devices even without using split screens or keyboards.
In Agenda, prepare the spot where you want the link to the email to appear and in mail on the iPhone/iPad, touch and drag the message you want to link to. Don’t let go. With you other hand, perform whatever gesture switches apps on your device or displays open apps (double tap the home button or four finger swipe works best for me but YMMV).
With the one finger still on the screen symbolising your dragged email, switch to Agenda and manoeuvre to the spot you want the link to appear and let go and voila, a link to the email materialises.
While it does require some dexterity, it sounds a lot more complicated than it actually is. Try it out, I use it all the time and find that it makes Agenda on the iPhone so much more useful
or on the ipad when I use it without a keyboard).
The same method allows inserting URL from Safari BTW.
I hope it helps