Under [Window Titles] add:

WindowXX=CS Weather Radar

Then add the following at the bottom of the file:

[WindowXX]
size_mm=525,386
window_size=0.44
position=2
background_color=1,1,1
visible=0
ident=11223
zorder=1
gauge00=WXR!wr_display, 81,32,364,279
gauge01=Captain_Sim.r001.P04!P04_radar, 0,0,525,386

XX must be the next available consecutive number.

I.e. Using the default C172 panel, if the last entry in [Window Titles] is Window05=GaugeName (Mini Panel in this case), then you should add it like this:

[Window Titles]
Window00=Main Panel
Window01=Radio Stack
Window02=GPS
Window03=IFR Panel
Window04=Landing View
Window05=Mini Panel
Window06=CS Weather Radar

And the [WindowXX] entry must match it like this:

[Window06]
size_mm=525,386
window_size=0.44
position=2
background_color=1,1,1
visible=0
ident=11223
zorder=1
gauge00=WXR!wr_display, 81,32,364,279
gauge01=Captain_Sim.r001.P04!P04_radar, 0,0,525,386

Hope this helps.