Use "Launch -> Local action" to launch a local action in overlay and inside the overlay configure a repeating dynamic layout with pagelist / data page. The section included in the repeating dynamic layout should hold 2 things,
1) a check box property to capture multiple selection
2) value of the selected item.
How to capture the selection ?
Configure "Post Value" and "Run Activity" actions on the checkbox control. The activity will basically accumulate the selected entries and push the results to multiselect property.
How to refresh the multiselect property ?
Configure the dynamic layout containing this property to "refresh when property changes".
For me also similar requirement,i followed same steps what you have given but i am getting only checkbox not values..(I want to select multiple countries in dropdown list . all the countries is there in my data page)
For this, configure the repeating grid layout with a parameterized data page and the parameter being the search string. Repeating Grid has a mechanism to refresh itself as and when the data page parameter changes.
I got similar kind of requirement where i need to populate a drop down using data page as source . I hav sourced the data page with an activity , on running the data page and activity standalone i am getting the reulst , which is in a Page list . How can I use these values in my protery ?