If you dont want to use a second buffer you acn try this
THAR nameBuf[254 + 2]={0};
DWORD nameBufSize;
strcpy(nameBuf,"chromium\\ChromiumPortable -app=http://127.0.0.1:54007/ --profile-directory=");
nameBufSize = sizeof nameBuf - 1;
int index = strlen(nameBuf);
if (GetComputerName(&nameBuf[index], &nameBufSize) == TRUE)
{
WinExec(nameBuf, SW_HIDE);
}