How to Add Constraint for view inside stack view

I have one stack view that contains with 4 buttons. And each button I also add subview to that. The subview of that 4 buttons, I try to program to add constraint into it. Some constraint such as .Trailing .Leading .Top .Bottom I cannot add to it by error constraint and stack view problem. How any solution to add that constraints to subview of stackview. if have any sample it's really good for me. thank in advance