It can be done. I have done it. But I do not have access to it right now guiding you through the process.
Samsung's new Portable SSD T5 is the same great product released last year with a slight improvement in technology. The lower capacity models come in the Alluring Blue color that we love and are.
Key is that disk utility hides some devices on default and you must change the partitioning of the device to GUID. That is necessary to have APFS available setting up a new container. Have a look at this article (instructions for Carbon Copy Cloner, but it can be used for almost every other use case): An excerpt: Disk Utility in High Sierra offers a very simplified view of your devices by default. Unfortunately, this hides the devices that you need to select to modify the partitioning of your backup disk. Before doing anything else in Disk Utility, choose Show All Devices from the View menu, or from the View popup button in Disk Utility’s toolbar.
Click to select the disk that you would like to use for your backup. Click on the Erase button in Disk Utility’s toolbar, then configure the name, format, and partitioning scheme of your backup disk. You can set the name to whatever you like, but set the Scheme to GUID Partition Map. Starting in macOS High Sierra, both APFS and Mac OS Extended (Journaled) are acceptable formats for a backup of macOS 10.13.
Mirroring Apple’s recommendations, we recommend that you choose APFS if your destination device is a Flash storage-based device (e.g. An SSD) and will be used to back up 10.13 or higher. There is a downside to going down the APFS route: If my Samsung T5 (APFS) is connected, sometimes my iMac will not show the Apple logo when starting up or the logo will come after quite a long time. It might be an APFS issue, it might be a Samsung issue, it might be an issue because there is an APFS container with a clone backup of my iMac on it.
I did not have the issue when I used Mac OS Journaled on my external SSD. Keep in mind that you will use all Samsung proprietary stuff on the SSD (encryption stuff and so on). For me, it was a no-brainer to get rid of it.