You simply can't: the coinbase tx of the genesis block is not spendable. You have to craft a blockreward function in a way that all your premine is created in the block #1. Then you have to find the solution for this block (the same way as you did for genesis block)
Yes you can configure your blockchain to mine 100% of total coin in first block and as nobody will be there mining your coin, you will get all of the coins in your wallet.
can you share some links for how to pre-mine 100% coin also how can I provide stake to that 100% coin.