See: Extract the coordinates of the boundary points and the value of a function on them - #4 by dokken
and change boundary_facets to the facet_tag.find(obstacle_marker), if facet_tag is the name of the facet markers.
You can then use entities_to_geometry to get the vertices of those facets, which can in turn be used with mesh.geometry.x to get the physical coordinates.
Do you mind me asking what you have in mind to do with these values?