Do you mean that I should insert manually this line :
<setting id="videoscreen.whitelist">0192001080060.00000pstd,0192001080050.00000pstd,0128000720060.00000pstd,0128000720050.00000pstd</setting>
To be precise, insert / edit the content of this XML tag into your existing empty XML tag.
The content is:
0192001080060.00000pstd,0192001080050.00000pstd,0128000720060.00000pstd,0128000720050.00000pstd
You can see four comma-divided values. The relevant numbers are before the point. For value one "0192001080060" is relevant. It means a resolution of 1920x1080 and a vertical frequency of 60Hz. Now you can insert all data, which your screen supports (see your manual).
The selected (in-use) value of the described whitelist is stored here:
<setting id="videoscreen.screenmode" default="true">DESKTOP</setting>
The value "DESKTOP" means 720p / 50Hz (last value of my whitelist). You can replace that value by any value of your whitelist.
Hopefully Kodi will not overwrite your edited guisettings.xml (store a backup). If it does, maybe removing the write permission for that file could help.