You will need to add the lines I have highlighted in bold to be able to set the value at which your trailing stop begins. You can create separate ones for long and short if you wanted that.

//trailing stop
trailingstop = 20
StartTrailingStopValue = <Set this value>

//resetting variables when no trades are on market
if not onmarket then
MAXPRICE = 0
MINPRICE = close
priceexit = 0
endif

//case SHORT order
if shortonmarket then
MINPRICE = MIN(MINPRICE,close) //saving the MFE of the current trade
if tradeprice(1)-close > StartTrailingStopValue*pointsize then
if tradeprice(1)-MINPRICE>=trailingstop*pointsize then //if the MFE is higher than the trailingstop then
priceexit = MINPRICE+trailingstop*pointsize //set the exit price at the MFE + trailing stop price level
endif
endif
endif

//case LONG order
if longonmarket then
MAXPRICE = MAX(MAXPRICE,close) //saving the MFE of the current trade
if close-tradeprice(1) > StartTrailingStopValue*pointsize then
if MAXPRICE-tradeprice(1)>=trailingstop*pointsize then //if the MFE is higher than the trailingstop then
priceexit = MAXPRICE-trailingstop*pointsize //set the exit price at the MFE - trailing stop price level
endif
endif
endif

//exit on trailing stop price levels
if onmarket and priceexit>0 then
EXITSHORT AT priceexit STOP
SELL AT priceexit STOP
endif