The target markers can only be applied if you move the mouse manually above a target and it only can remove this same marker if you move the mouse above that target again.
If no target below your cursor -> marker will be set to yourself! (or if same marker was set to an enemy already it might remove it on first applying without showing it at yourself)
Addons cannot circumvent this and there does not exist any good API to fullfill your request.
My addon FCOAccessibility enbales automatic target markes on enemies that got into combat and dmg was done to.
https://www.esoui.com/downloads/info...ssibility.html
It was created to help visibly impaired players so that their group leader automatically sets the target markers on the enemies and the visibly imparied players get the target markers at the compass, and if they turn around the screen reader detects that and reads it to them so they can find the enemies easier.
Mayeb this can help you too, to auto set markers on enemies in fight -> only the group leader can set the markers and he obviousl needs to run the addon then
For other purposes check the following addons:
https://www.esoui.com/downloads/info...etMarkers.html
https://www.esoui.com/downloads/info...utoMarker.html