First you have to add a level sequence.
Add the actor that you want to animate or keyframe in that level sequence.
Add the animation or the keyframes you want in the sequencer for this actor.
Right click on this actor in the viewport. ( This step is crucial because if you do not, your actor will not appear in the blueprint level when you will search for it)
On top, just below the Build tab, you have an Icon with 3 little squares that are connected.
Click on it then click on « Open Level Blueprint ».
Inside the level blueprint, right click on the grid and the actor name must appear.
Click on it to add it.
Then right click on the grid again, and search for the node that is named « Create Level Sequence Player » and click on it.
Inside that node, just under « Level Sequence », find the name of your level sequence and chose it.
Then, in the grid, connect the « Event BeginPlay » node to the « Create Level Sequence Player » node you just created.
Now you must click on « Compile », the button is on the top of the screen, to the left side.
Now you are good to go.
When you render your scene, the actor move as keyframed or animated in the sequencer.