Implement the url redirector widget in app A. Implement the deeplink module in app B. In app A construct the url for app B, something like https://appB.mendix.com/link/deeplinkname/parameterstring In app B configure your deeplink with the 'deeplinkname' and select a microflow with optionally the parameterstring and run that flow to open the form. The deeplink will handle the serving of the login page on app B.
Hope this helps you further in implementing a solution.
If you always want to create a new entity in App B, you can use a deeplink without any parameters, and create the new entity as part of the logic in the deeplinked microflow.
By the way, I would expect SSO to be a requirement here. It seems user unfriendly to ask them to login when navigating to App B.
If you would need to create new entities based on information from App A in App B you could also use the an app service to exchange information between both Mendix applications: https://world.mendix.com/display/refguide5/Published+App+Services