I do not see anything in my Bangkok bank App to allow me to do this.
On the landing page upon opening the banking app, left-hand side under 'Quick banking', there's a 'Top up' button. Once you have used it, it remembers the number you last topped up. If you want to top up a different number, reselect the carrier button rather than the 'remembered' top up button.
If you have the AIS (or DTAC, or TRUE) account app installed on your phone, you can top-up from there. If you chose to top-up from there, it will launch a login to your banking app as above.