Just participated in this seamless purchase experience:

1) Register with WAVES Platform for an address (required field input)
2) Log-in to Prime-Ex Portal
3) Decide how many PEX you need, payment mode: WAVES / ETH / BTC
4) Input WAVES Address & click 'Buy Tokens'
5) A unique WAVES or ETH or BTC Address will appear for you to pay based on your selected payment mode.
6) Go to your personal wallet in your exchange and send either WAVES/ETH/BTC to the unique address.
7) Wait a bit ............
View transaction history in Prime-Ex Portal: You confirmation of purchase should appear.

Just sharing my experience, as each ICOs has a different approach. All the BEST!