I have a global variable: Globals.speed_multiplier and when pressing shift it should go from 1 to 20
....

export(Curve) var acceleration_curve
	
Globals.speed_multiplier = acceleration_curve.interpolate(lerp(acceleration_curve.min_value,acceleration_curve.max_value, 0.1 * delta))

...
that doesn't work, I don't know what I'm doing or how anything of this works.
In Unity there is a curve.Evaluate but Godot doesn't has this.

EDIT

Nevermind, figured it out.
created a float called speedup, because I'm great at naming variables and simply did this... maybe I should clamp this number, else it would increase infinitely.

	if Input.get_action_strength("Boost"):
		speedup += 0.9 * delta
		Globals.speed_multiplier = acceleration_curve.interpolate(speedup)
	elif Input.is_action_just_released("Boost"):
		speedup = 0
		Globals.speed_multiplier = Globals.speed_multiplier_cache