I would like to generate a number between 300 and 450
I tried to use poissrnd but I couldn't generate a number between the two values.
A poisson distribution is defined over the range [0, Inf] and has only one parameter lambda (mean and variance). You cannot have a set of numbers that follow a poisson distribution and at the same time is restricted to a smaller range than [0 Inf].
You could generate random numbers that follow a poisson distribution with mean (300+450)/2, but you are guaranteed that with enough samples some of these will be out of the interval. Otherwise it is not a poisson distribution anymore.