Blue birthday is a shorter web drama but thats also similar
There the FL tries to save the ML in the school days and has come from the future just like in Lovely Runner

In Park hae jin starring 'The killing vote ' also there's people voting to either kill or save the target decided by gaetal for that vote.
The application opens on the phones of everyone.
But this show is not horror or supernatural more of a thriller.

The first two episodes totally give the legend of the blue sea vibes. I'm assuming its because the female lead has to adjust being in a different era which was similar to that kdrama.