Adding an element to a listview in viewmodel

I am trying to make a tab page with a list view. I have two questions.
First) I am trying to make each page have a list view that receives Item source from list of view model. Do i add the elements directly to the list in view model or from the page?
Second) Is there a way to add an element form one page to another by clicking a button? (So like a button on scheduler adds the selected schedule to the personal agenda)
I've been researching and reaching out for help and actually even some folks helped me out, It seems not working. Should I use observable collection on model view or just a list and make it into observable class in each page? please help!