Which trigger activates when Edit button is pressed

(2016 NAV) In Page i have field from two Text variables ( 250 length of each) for exapmle variable a and b. When field text is above 250, the above field part is assigned to variable b. Is all working fine, but this field is always editable, i want to do it editable when action button "Edit" is pressed. So I created bool variable (to field option EDITABLE) and want to assign TRUE to it when button "Edit" is pressed and I don't find where I should put it. Maybe someone know in which trigger i have to assign my variable?