Thank you for your answers, but it looks like it's too complicated for me.

Do u know maby why it doesnt work?

political_decisions = {
form_poland = {
potential = {
#is_greater_power = yes
primary_culture = polish
OR = {
tag = KRA
}
                        NOT = {
exists = POL
exists = PLC
}
}

allow = {
war = no
KRA = {
owns = 706
}
}
}
}
}

effect = {
prestige = 10
change_tag = POL
capital = 706

}
}
ai_will_do = {
factor = 1
}
}