Easily done, with a PHP Script. You'd need to use the use of timestamps if you wanted it to be dynamic timings, so 24 hours from when the user last opens it. Although, the easier option would be too set the date yourself, so its a daily reward. I'll include an example of how it was done on Pure...
My name is Miss Fatima and please i want us to be good friend but you have to contact me with my email address [ fatimadel23 @ hotmailcom ] so that we will know ourselves very well and i will send you my pictures too.
Thanks and take good care of yourself.
In server_settings you'll see welcome_message then a column for the value next too it, in here write your welcome message you wish users to see. If this cell is null no welcome message will appear. See the screenshot included if youre stuck.