URL scheme for sending SMS or iMessage to multiple recipients on iOS

Can be used to send a new single or multi-person message (optionally with the message included in the URL¹), or to open a previous conversation with one or more recipients.

Phone numbers have to be without space characters or can alternatively be URL encoded, and can be with or without country prefix. Looks like some SenderID² names are supported too, possibly depends on if it's an ID that can receive messages or not. To specify multiple recipients, separate the numbers (or names) with a comma, ex. 1-800-692-7753,1–800–854–3680. If you want to include a message, it needs to be URL encoded.

sms://open?addresses=numbers or names separated by comma

or with a message attached:

sms://open?addresses=12345678&body=My%20URL%20encoded%20message

¹ "SMS WatchKit multiple number delimiter"—stackoverflow.com
² GSM SMS alphanumeric SenderID