I do this in my telemedicine app 
- glide side - button with the parameters to pass to integromat
- eg tel, message, time to send
- integromat scenario - 2 separate ones
A - webhook gets values and stores in a Data Store (set with 1mb side, structure matches the we hook parameters, store the time value as a string)
B - scenario running on scheduled basis
i search Data Store - all records (best)
II - filter based on time to send compared to now (format as time)
Iii - click send or Twilio - Tel and message will be from the data store
Took me a couple of hours to figure out looking at some videos.
Good luck!