Of course there is an easy way to type in the address into the timer box, in the space to the right of the word "Timer" where the "?" first occurs. For me, that is the ONLY way I ever put in the address.
When you first insert the RTO box, the address space contains "?". If you click once there and just press enter, it will change to the first timer address, "T4:0". Then you can double-click on "T4:0", highlight the 0, and type in whatever timer number you are using, from 0 to 99 in the LogixPro Simulator, or from 0 to 255 in the real RSLogix500 program.
If you want to take longer and fool around, you can drag an address there (f you can find the correct one). However, if it is the first time you have used a specific timer, the address will not exist in your LogixPro program rungs, so it will be difficult to find the address. In LogixPro, you can right-click on the timer box, select "Data Table", then go to Data File T4, pick your timer address, and then drag it into your timer box. It seems less work to me just to type it in (I learned to type before computers were invented), but maybe the dragging is easier for you younger guys.
In the real program RSLogix500, for the timer box address, you can go over to the menu and under Data Files, click on the "T4 - Timer" menu, then select the first one (the only one that will be there in a new program) and drag the label over to the timer box. What a lot of work! Also for the next and all other timers, they will not show in that menu (unlike LogixPro) because they doesn't exist yet.