If you ever want to setup a Message of The Day for people logging into your vCenter Server (maybe you have a organisational policy, or for identification if you use multiple vCenter), log on to the vCenter using vSphere client, Click on Administration, select Edit Message of The Day (see image below)
Once the new window opens, type your desired message and click OK
If you want to setup MOTD for each ESXi, you can easily do so on ESXi version 6.0 and up, from within vCenter, by selecting the host, going to Configuration Tab, Software, Advanced Settings and to go Config -> Etc.
The difference between the 2 fields is that the Config.Etc.Issue one will show up when you login via SSH between the username and password fields, while the Config.Etc.motd one will show up after you succesfully login.
If your ESXi is not managed via a vCenter server, you can edit the above 2 fields while logged into SSH.
To do that, you need to type in the SSH window the following command:
Depending on which one you want to edit.
Press i on your keyboard to start insert, and then start typing your message. Once finished, press Esc to exit insert mode then type ZZ, which will save and exit.
Make sure you restart your SSH service using /etc/init.d/SSH restart .