Automation Blocks - Markers

Markers in Ae are effectively just keyframes (they just look different in the UI). Whereas a position keyframe is a keyframe of the property "ADBE Position", a layer marker is contained in the property

property "ADBE Marker" of a layer. But you can use the Get Attribute of Property block and "Set Attribute of Property" with it in the same way as for other properties.

The community library contains some examples in the section
Layer Properties/Markers
for example this one:

The tricky thing is that when you get the value of a marker, you obtain a big json string, containing the marker name, comment, type etc. And when you want to set it, you need to set such a string. So it is not really well supported, but definitely possible.

Of course, you can also always use the Execute Code block, to work with the Ae API directly.