I figured out it myself, and yes, you are right:

tstr = GetTabTextFromArea(LineNr,ColNr,a);// a string longer than 255 chars
ColNr = ColNr + 1;
SetAreaZeroSize(area_t);

AddTextToArea(trim(tstr),area_t);
AddAreaToTextField(area_t,NoterDN);//all chars are copied to Noter
//AddToText(tstr,NoterDN); only first 255 chars are copied