How to add A menu to item to online store Existing Navigation (Not in Admin Login).

[ManiKanta]

 Hi, I am developing public App,

when Installing My  App, it will create a Page with Iframe with API call. 

so Now I want to know how to add a page as a menu item in already existing navigation in the Online store.

Note*: I want to add a menu online store, not in the Admin login.

Please let me know, how to add a page as a menu item in existing Navigation in the Online store without manual interaction (Ie: using API).