ExileClient_gui_hud_event_onKeyUp.sqf
case 0xDB: ///* Left Windows key */ change to 0xDC for /* Right Windows key */
{
    switch (ExilePartyEspMode) do
    {
        case 0:     { ExilePartyEspMode = 1; };
        case 1:     { ExilePartyEspMode = 2; };
        case 2:     { ExilePartyEspMode = 0; };
    };
    _stopPropagation = true;
};
https://community.bistudio.com/wiki/DIK_KeyCodes
DIK KeyCodes
Or you could just add another one below, like this
case 0xDB://left windows key
{
    switch (ExilePartyEspMode) do
    {
        case 0:     { ExilePartyEspMode = 1; };
        case 1:     { ExilePartyEspMode = 2; };
        case 2:     { ExilePartyEspMode = 0; };
    };
    _stopPropagation = true;
};
case 0xDC://right windows key
{
    switch (ExilePartyEspMode) do
    {
        case 0:     { ExilePartyEspMode = 1; };
        case 1:     { ExilePartyEspMode = 2; };
        case 2:     { ExilePartyEspMode = 0; };
    };
    _stopPropagation = true;
};