Post to Sprout Forms plugin from an app or another website
Assuming you're using a hidden action input, then sproutForms/entries/saveEntry should be correct.  You can see that in the Craft docs here: https://docs.craftcms.com/v2/plugins/controllers.html#posting-to-controller-actions

If you're not using that input, or you're constructing the URL to post to manually, then it will likely be actions/sproutForms/entries/saveEntry instead, assuming you have set a custom actionTrigger in your general config.