Just one of those things you have to know… It actually isn’t too, bad. Each major android version gets a letter, a number, and a name. I have no idea why they need three things. The letters are assigned alphabetically, so N, O, P, etc. The numbers are increasing, with integers representing new major versions, and decimals minor versions. So 7, 8, 8.1, etc. The names are sweets that start with the same letter as the version letter. So nougat, oreo, pie, etc. (Pendant alert: letter versioning used to not always represent integer number versioning, but it has for the last several years.) Anyway, here is a list. It’s not the stupidest versioning/naming system I’ve seen.
On top of the Android letter/name/number there also might be another name from the phone maker. For example, Samsung One UI 1.0.
You say you have a Samsung 8, do you mean the S8? I have a Samsung S8 running Android 9, and this is what I think you can do:
Make the navigation bar with the three buttons appear at the bottom of the screen. It might be there, or you might have to swipe up to get it. Tap the app switcher button. On mine that looks like three vertical lines, and is the left most button.
Tap the “Close All” button that appears near the bottom of the screen. That should return you to your home screen.
Open your texting app, however you normally do that.
Bring up and tap the app switcher button again. It should show your texting app on the screen. Tap the icon for the app, which should be just below the search bar at the top. That should bring up a menu. From there, tap the “Open in split screen view” option. That should bring you back to your home screen, with your texting app rolled up at the top.
Go open your login app. That should put it in a split screen view with your texting app at the top, and your login app at the bottom.
Reasons all of that might not work: Your “Samsung 8” might be different than my “Samsung S8 running Android P/9/Pie/One UI 1.0/Samsung Experience 9.0”. The app you’re trying to login with may refuse to work in split screen.