Constrain one coordinate of a point via Expression in Fusio?

I'm working on a macro (in Resolve's Fusion module) that features (among others) one polyline, where the first line segment should be kept horizontal, while the user may freely move the first point and move the second point horizontally.

However when I try to add an expression to the Y coordinate only, a Point() with two coordinates appears. I can enter a formula for the second coordinate (Y), but I'm unsure what to do with the first:
If I leave the number displayed fort he first coordinate, the point cannot be moved by the user any more.

So how would I implement what I wanted?