I need to install more applications - can I use my external HD (samsung T5) and add an additional applications folder? I have loads of space on it and it the speeds are great.
Yes, you can use an external drive to keep (install) apps.
You should be fine. You don't need to specifically create a folder named Applications on the external drive. Any name or directory hierarchy should be fine.
macOS apps are generally made available as bundles (.app extension) which is technically a directory appearing as a single file to the user (with a specialized structure). An app bundle could be placed anywhere on secondary storage and the app can be used without any difference.
By default, apps are installed in boot volume's
/Applications directory and are automatically made available to all the users on macOS. Keeping an app bundle in a folder/drive only accessible to you limits the app available to you.
On a related note, it is worth mentioning for the macOS newcomers that installing/uninstalling an app in macOS is as simple affair as copying/trashing the app's .app file.