It is important to help your players complete their goals. In the image above you see how 4 different poker sites, Partypoker, PokerStars, Full Tilt Poker and Unibet (Microgaming), and their implementation of the Buy-in dialogue.
When a poker player sits down he needs to bring money to the table. How much he can bring in depends on his bankroll and the table limits.
In all the examples above you see that they have done a couple of different implementations to the same functionality.
You should base your design on stats!
You could even personalize it based on the player behavior. For instance if a player always plays on one table and brings in all his money the you should pre-select “Maximum Buy-in” but if a player plays on more tables then you might want to select an amount of his bankroll.
In the situation where you don’t bring everything in then the amount should be selected so that the player can type in the amount right away if he wants to change it (2 of the poker sites above do that).