U just check the box with goals in the substitution U want to made!
U can find in the 1st box in goals part choices like > , >= , = , =< & <
then U can find another blank box where U insert the no. U want!
That means for example if U chosen >2 that the sub will take place once U score ur 3rd goal!
another example, >= 1 means that the sub will take place once U score a goal!coz of equal sign!
Don't try < (less than) coz that will make the sub take place in 1st min of the game coz it's still 0 goals U scored

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wish it was helpful!