It depends on your immediate plans for the milkshake and how you want to preserve it.
If you want to consume the milkshake within a short period (a few hours), you can simply put it in the fridge. The cool temperature of the refrigerator will help keep it chilled and safe to drink.
However, if you're not planning to drink the milkshake right away and want to save it for a longer period (a day or more), putting it in the freezer is a better option. Freezing the milkshake will halt any bacterial growth and keep it from spoiling. When you're ready to enjoy it, you can take it out of the freezer and let it thaw a bit to reach a drinkable consistency.
If you choose to freeze a milkshake, it's essential to use an airtight container to prevent freezer burn and preserve the flavor and texture of the milkshake.
In summary, if you plan to consume the milkshake within a few hours, store it in the fridge. If you want to save it for a more extended period, place it in the freezer.