How to verify signature when using WidgetInvite/WidgetSuggest?

Question

Classmates

I'm trying to open WidgetSuggest in a WebView in a mobile application. It redirects me to a page with a "BAD SIGNATURE" error in the url. In this case, the problem occurs only if you use the st.return parameter, without it everything works as expected.
Similar code with WidgetMediatopicPost also works.
What can be wrong?