Ok, this is NOT an elegant solution, but if you have operations people breathing down your neck you can use this as a temporary rig to get something going while you work out a better solution.
As noted, you really want to find an XMPP bot to do this. Google XMPP bot and you'll find lots of solutions.
However, if you have a client machine (or VM) sitting around doing nothing and you want to mash out something quickly while you figure out the XMPP bot and how it works...and, I assume this is a windows client, you can use Sendkeys and a vb script to do it.
Note, once you set focus to the conference window, focus cannot be lost from that window. I didn't take the time to have focus set in the code (though, it is something you could modify the script to do if you wished). Rather, it would require that computer sit there...with the conference room in spark with the window in focus (selected).
Instructions to use:
Open a new notepad document and name it what you want.
Change the 3 letter extension to .vbs
Open it with Notepad and paste the following code into it. Be sure to read my notations. All notations start with an apostrophe ' . I mention commenting and uncommenting lines of code. To uncomment, delete the apostrophe. To comment, add an apostrophe to the front of that line.
Code below --->>>
Set WshShell = WScript.CreateObject("WScript.Shell")
dim infiniteloop, Message, LoadPause, AfterMessagePause, PostPause
infiniteloop = 0
'You'll want to modify LoadPause to give you time after you double click on the vbs file to start the script to click on the appropriate conference window. Once focus is set on this window, it cannot be removed, or the script breaks
LoadPause = 5000
'Put your message below
Message = "Put your Message Here"
'In my limited testing tossing this together for you, 1000 worked well for this, but you may need to up it when you do your tests. If you don't leave this pause in, then the script will send the message multiple times to the conference window, as the script is moving faster then Spark.
AfterMessagePause = 1000
'This is set to 30 seconds (30000 milliseconds) for testing. For 15 minutes, you'll want to modify this value to 900000.
PostPause = 30000
Wscript.Sleep LoadPause
'Loop message
'Comment out the line below when done testing to turn on an infinite loop.
Do while infiniteloop<5
'Uncomment the line below when done testing to turn on an infinite loop. If you ever want to stop the loop, you'll need to open task manager and kill the Windows Scripting Host process.
'Do while infiniteloop=0
WshShell.SendKeys Message
Wscript.Sleep AfterMessagePause
WshShell.SendKeys "{ENTER}"
Wscript.Sleep PostPause
'Comment out the line below when you are done testing. When you do this, the script will run indefinately.
infiniteloop = infiniteloop + 1
Loop