*A lot of people are reporting issues that no trials have been showing up in recent weeks. When I can, I'll investigate this to see what I can find for people!*
This one is easy but a bit confusing to find.
Trials are types
of seasonal championships, which are the red shields. You will have needed to qualify for the Horizon Roster to get access to these.
However, not all seasonal championships are trials. The one you're looking for is with a lion's head.
If you're still struggling to find it, to get around this, I would recommend quitting Forza Horizon 4 and re-launching it. When you load in, it should show you a menu of all the seasonal championships available to you before you can access the full map. Pay attention to these prompts,
as when it goes through them all one of them will say "trial" in a way much more obvious than just reading the map.
Just matchmake a session, complete the two races (its pretty difficult, but placement doesn't matter) and you'll get your achievement (in this instance you'll, of course, need to purchase or hire a Lamborghini.) I'm not certain if you need to be a certain level to unlock the championships, but I'm around Level 4 in all the racing disciplines so I don't think you need to be a high level in that week's trial discipline unless something changes going forward. Following the above method should always allow you to "find" your seasonal trial championship