Screeners are much the same as indicators, with just a few exceptions:

• do not support graphics
• do not return any data, they simply highlight the instruments that meet the conditions
• have a data history of just 254 bars, with IG (PRT offer 1024 bars, though).

That said, this indicator:

Sma100 = average[100,0](close)
Signal = 0
IF close CROSSES OVER Sma100 THEN
   Signal = 1
ELSIF close CROSSES UNDER Sma100 THEN
   Signal = -1
ENDIF
RETURN Signal AS "Crossover"

can be easily converted into a screener by simply:

• replacing the last line with the keyword SCREENER (and optionally adding the CRITERION)
• replacing -1 with 2 or any positive value of your choice to tell long from short signals

Sma100 = average[100,0](close)
Signal = 0
IF close CROSSES OVER Sma100 THEN
   Signal = 1
ELSIF close CROSSES UNDER Sma100 THEN
   Signal = 2
ENDIF
SCREENER[Signal](Signal AS "1=long, 2=short")

IG only supports 254 bars, for all TFs, that is 254 minutes, hours, days, and so on…

1024 are supported by PRT from their own website, not with IG.

If any indicator needs more than 254 bars, then it will return incorrect data.

Be warned that some indicators need many more bars than the periods in brackets. EMAs can have, at most, about 70-80 periods, because of their inner calculations.