Here's one way:
NamesDefaultToHere(1);
// ******************************************************************************************
// Given a column reference, sets spec liomits property
// ******************************************************************************************
SetSpecLimits = function({c, lsl, usl, tar},
cmd = Expr(c << SetProperty("Spec Limits", {LSL(expr(lsl)), USL(expr(usl)), Target(expr(tar))}));
eval(evalexpr(cmd));
);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
SetSpecLimits(Column(dt, "height"), 60, ., .); // Set LSL only
SetSpecLimits(Column(dt, "weight"), 80, 120, 100); // Set all three values
Use 'Help > Scripting INdex' and 'Help > Books > Scripting Guide' to help figure out how it works.