OK, I finally figured out how to do this! To set up a timed transmission, do the following:
In FLDIGI, right click on a macro button, then paste the text you see below. Note that the "60" is the amount of time in seconds between transmissions and you can set it to any value that you want:
<TX>
<TIMER:60>
(Insert your transmission text here)
<RX>
Once you click on the finished macro, the transmission will automatically transmit every 60 seconds, hands-free.