Just add:

set stop %loss 0.3

to the bottom of the strategy and it will sell when price hits 0.3% below your buy price.

Alternatively you can add:

if onmarket and close <= positionprice * 0.997 then
sell at market
endif

and it will sell at the close of any candle that closes 0.3% or more lower than your buy price.

You should check that 0.3% is not too close to price as your broker will have a minimum stop distance and reject any order that is too close and possibly stop your strategy. You can use use MAX to try to ensure that no order is sent that is too close although sometimes brokers can add a big spread and a big minimum stop distance during high volatility so this is not totally foolproof.

For example if the minimum stop distance is 5 then the following will ensure that the 0.3% order is never closer than 5.

minstopdistance = 5
minstopperc = (minstopdistance/close)*100

mystop = max(minstopperc, 0.3)

set stop %loss mystop

defparamcumulate orders = false
 
myrsi = rsi[14]
 
if myrsi < 30 then
buy 1 contract at market
endif
 
if onmarket and myrsi > 70 then
sellshort 1 contract at market
endif

set stop %loss 0.3