Full screen of your view (for a more easilly view of the Snap Perp on the curve) because when you zoom your first click on the curve can be becoming out of the screen ! )
Disable Grid Snap and move your mouse with the helper line untill see Perp Snap ;)
Re Enable Grid Snap if needing

You can draw this Helper line before the first click of the Line, after the first click of the Line on the curve...(and before the last click... of another function used)

So you have the choice to click the first point of the Function Line on the curve or not! ;)

Here the first click of the Line function is not made ---just the first click of the Helper Line
--- I am waiting the second fixed click of the Helper line for draw the two clicks points of my line! --- suspense :)
And because I can move (If I want) my Helper Line on another point of the curve and Reorient it for a New Perp Snap!!!

Like this I can draw any number of perpendicular Helper lines along my curve for choice the better one for my wish !