See the documentation. /link/mystring invokes the deeplink 'mystring' without arguments, so it should map to an microflow without arguments.
For links like /link/mystring/myargument you can either use 'myargument' as string argument in which case the microflow should have an string argument, or you can define that 'myargument' maps to an object attribute (for example System.User.Name) in which case the input argument of your microflow should be a System.User.
In the latter case deeplink will do the lookup for you. The advantage is that this will be checked and return a 404 if not found, before the whole client is loaded.