We're still in the trial phase of Lansweeper, but I wanted to see if it was possible to integrate Splashtop into the custom actions. Splashtop has limited documentation on how to connect via a command/shortcut, but I was able to adapt it to work in Lansweeper. NOTE that this custom action works only for assets that have Splashtop Streamer installed, and the computer you're using to access LanSweeper is Windows and has the Splashtop Business client installed.
It turns out that the "account" part of the URL isn't needed if your Splashtop client is already logged in, I suspect it would be useful if you have multiple Splashtop accounts signed into a client - for example, I have my Work Splashtop acccount, and my personal one - and I can switch between them in the File menu. If you don't provide the Account parameter, I suspect it just assumes that the computer you're connecting to is in the account you're logged into.
The trickiest part of this was making the MAC Address that Lansweeper gives you work with what Splashtop expects (no colons). I ended up using a couple lines of Powershell to manipulate the string and then call the URL. MAC also appears to be the only valid parameter besides Account, I tried to brute force some other way to use the hostname or asset tag or something - no dice.
I figure this could probably be re-written to work in MacOS or Linux terminals using their own string manipulation features. If anyone has any ideas on how to make this cross-OS compatible, I'm all ears!