You can do it through tweaking the values of
winposstr inside the rdp file itself. Here is a detailed explaination
Calculating the Values for winposstr
The numbers that you will use depend on how many monitors you have, the screen resolutions of each, and which monitor you have selected to be your primary monitor.
Left edge position determines the monitor
The first of the last four numbers, which is the most important, indicates the position of the left edge of the window. If your primary monitor is in the center and you have monitors to the left and right, you will use a negative number for the monitors to the left, and a positive number for the monitors to the right.
For example, if you have two monitors to the left that each have a resolution of 1680 x 1050, use -1680 to place the session window on the monitor to the immediate left, and double that (-3360) to place the window on the leftmost monitor.
If your primary monitor has a resolution of 1200 x 1920, use 1200 to place the window on the monitor on the immediate right. This works because that pixel position is where the primary monitor leaves off (and the monitor on the right begins).
If you have two monitors on the right with a resolution of 1680 x 1050, and want to place the window on the rightmost monitor, simply add 1200 to 1680 to get 2880.
Top and Bottom Edges
The other numbers only really come into play when the window is restored (not maximized) and can be adjusted to taste.
The second number (third from the end) indicates the topmost edge of the window, so you can leave this number at 0 unless you have special requirements. The fourth (last) number is the vertical resolution (=bottom) of the window. You can leave this at the maximum vertical resolution of the monitor, as in the examples above.
The right edge of the window is expressed by the next to the last number. Unless you prefer otherwise, this can be set to the same size as the maximized window.
║ Monitor ║ Right Edge Calculation ║ Winposstr Value ║
║ Leftmost ║ -3360 + 1680 = -1680 ║ winposstr:s:0,1,-3360,0,-1680,1050 ║
║ Immediate left ║ -1680 + 1680 = 0 ║ winposstr:s:0,1,-1680,0,0,1050 ║
║ Immediate right ║ 1200 + 1680 = 2880 ║ winposstr:s:0,1,1200,0,2880,1050 ║
║ Rightmost ║ 1200 + 1680 + 1680 = 4560 ║ winposstr:s:0,1,2880,0,4560,1050 ║