First, go Encyclopedia > Game History and click on the run you wants.

One of the information you can have is your seed.
(bottom right of the screenshot)

After completing A-rank, you unlock the possibility to start seeded runs
There is a box to enter your seed just under "Start Run"

In this box you can enter a seed retrived from your history or somewhere else.
But not only. Rogue: Genesia use only numbers in its seed generation, but you can also use letters in your own seed.

Create a new run (you can do it with a challenge)


Now, before choosing your path display the console (it's the key under ESC)


"startrun" is the command you want and it need two arguments
First is your Difficulty: F is 0, E is 1, ..., A is 5
Second is your Seed: You take it from your game history or from somewhere else.

So if I want to redo a B run with the same seed from my exemple run my command is:
startrun 4 1027632128

Important: if you wants to do a seeded challenge you need to create a run in your challenge before applying the command, of course the world argument needs to be the right one. I you choose the wrong world argument then you will do your challenge but in a wrong difficulty.

Valid with Enter and you have now a run with the seed you wants.