You can find the NumHop script dialog under Tools->Num Hop Script or press (Ctrl+Shift+N). There is one NumHop script for every system (subsystem) in your model. The following is supported:

# The hash sign mean that the rest of the line is a comment

# Assign a script local variable
a = 1

# Multiple expressions on same line with ;
b=2; c=3

# Supported operators are:
= + - * / ^ < > | & (...)

# Example on valid expression
a = (a + b)*c/a^c

# Access a component parameter
a = ComponentName.ParameterName

# Assign the same way (a can be a numeric expression, variable or value)
ComponentName.ParameterName = a

# Built-in math functions, a subset cmath.h, see  https://en.cppreference.com/w/cpp/header/cmath for details
# Taking one argument
abs acos asin atan ceil cos cosh exp floor log log10 sin sinh sqrt tan tanh

# Taking two arguments
atan2 pow fmod min max

b = cos(pi/4)
c = min(a, b)

Below is an example:
Consider a model with two metal rods where length, diameter and density is known. You can set the known parameters and then automatically calculate start values and parameters such as area and mass

# Global parameters
pi = 3.14159265358979

Rod1Length=0.6
Rod2Length=0.8
Rod1Diameter=0.070
Rod2Diameter=0.080
density = 7850

StartPos = 0.1

# Set and calculate default start values

Rod1.Length = Rod1Length
Rod1.Area = (pi*Rod1Diameter^2)/4
Rod1.P1.Position = StartPos
Rod1.P2.Position = -(Rod1.P1.Position+Rod1.Length)

Rod2.Length = Rod2Length
Rod2.Area = (pi*Rod2Diameter^2)/4
Rod2.P1.Position = -Rod1.P2.Position
Rod1.P2.Position = -(Rod2.P1.Position+Rod2.Length)

Mass1.m = Rod1.Area*Rod1.Length
Mass2.m = Rod2.Area*Rod2.Length