I'm no expert coder or anything - in fact, I'm pretty sure this isn't the best way to do it - but I got this to work.
You'll want to add a script section in the event that looks like this:
You'd probably want to run ExtendText from the main project folder, as that you give you more room to add the code.
Now, how it works.
You'd want to set that first line 'random3=rand(5)' is set to the same number of Pokemon that you want. You'd be changing the number in parenthesis. So if you have ten Pokemon on the list, then set it to 'random3=rand(10)' and such.
Then the next line 'if random3==0' tells it what to do if the random number selected is 0 (these numbers start a 0 instead of 1, so you'd only go up to 4). In this case, it would be to give the player a Bulbasaur. The next line, 'if random3==1' tells it that if the number selected is 1, then the player gets a Charmander. You'd want to do that for each Pokemon that they player can get, going up by one each time.
Then make sure that you have that nice 'end' at the very bottom, which tells the code to end.
I hope that helps - I'm not a coder by a long shot, but it's something I was able to piece together that functions. Someone more experienced might be able to give you a much better method of doing it.