please help!!!

how do I buy just a portion of a stock? On other trading apps I can enter a monetary amount, and it will give me a percentage of the stock. For example, 10.00 on a stock that costs 100.00 let's me have .10% of it. This is the only app I have that I can't figure out how to, and it's very frustrating. please help me to do this, or let me know if it's not possible.