You can find a window’s class name by running xprop from the command line and clicking on clicking on target window (xprop displays the window’s X properties); then search the xprop output for the WM_CLASS(STRING) property, which will show the window class name you should use with picom.
Tips:
Use xprop | grep «WM_CLASS» instead of xprop to show only the window class name.
Speaking anecdotally, if the two values of WM_CLASS(STRING) are different, use the second one.
