As far as I know, this should be possible using the deeplink module in combination with an SSO solution. The answer to your specific question:
? How can I use the UserAccount of one application, to automatically let the user sign in into the deeplinked application in which user also has a UserAccount?
Is: you can't, but you can fake it. If both apps have SSO and the same user accounts, the app you are linking to can perform SSO with the user requesting the link, before actually showing the contents. In practice, to the user it will appear as if the link just takes longer to load, because of the SSO.
In terms of Mendix app store modules:
To summarize:
? How can I use the UserAccount of one application, to automatically let the user sign in into the deeplinked application in which user also has a UserAccount?
To let a user use a mendix App it needs a valid session in that app. If the user is logged in Portal it is not automatically logged in in BackBone. You need a mechanism to achieve that. This can be done with a unique session id that is passed from Portal to Backbone. Backbone needs to against Portal wether this is a valid session and if yes create a session in backbone.
The last part you have to write yourself in java but you can use the loginhelper from SSO which does almost the same.
I ran into this same issue. No matter what, the deeplink in app2 was created and I was sent to it, but i wasn’t being logged into SSO despite it being set up. It was being created by an anonymous user and loaded as that anonymous user.
After ripping my hair out for days, I realized the key is in the Deeplink config: ALLOW GUSTS *must* be turned off. This is something that isn’t mentioned anywhere in any documentation and I’ve found many posts here in the forums about this but no one mentions this one key setting that is neccessary. It is a *little* bit slow to open the deep link but eventually it works. it tends to take 5-10 seconds.
My question is though, what if you wanted a deep link to work this way, but also wanted specific deep links to be available to guests (people outside your company viewing a read only report). Is there a way to accomplish both?