Is there a way to make the screed crack in a straight line in a certain place?

An emergency exit with a vestibule and porch. The whole thing falls from the building. Now there are three or four centimeters less. You can see the street.

The floor tiles are on the divergent joint between the building and the porch. The tiles are peeling, hanging down and broken in some places. Underneath is a multi-layer screed that has spread in a "crooked" line (the red line in the photo).

It had been peeled off. It became clear why it did not crack in a straight line. There are a few things hanging from the rebar growing out of the porch.

I want it to crack along the blue line (in the photo) in the future. Cut a joint between the tiles along that line, cutting it to the right size.

Initially, the idea was to apply soap or wax to the porch tiles.

You could lay a plastic sheet along the porch beam and drive some rebar into the screed floor of the building at the corners. You'll get something more or less monolithic – it will crack, hopefully along the beam. But you will not get the film up to the tiles – there is still about 6-7 cm of screed to be laid from the slab level to the tile level. In this case, the screed will crack uncontrollably.

You can use linoleum instead of foil. Bend the part horizontally and bury it. And along the beam. It is rigid – you can raise it to just below the tiles. It is approached with a screed on both sides.

Can you tell me how to make sure the grout doesn't stick to the porch beam (slab)? Has anyone done something like this before or are there standard solutions?