Mulitple texting not in your contacts

I remind people that their car is due for a service via text. They are not in my contact list so have to manually input their mobile number. There is no comma on the keyboard so I can't add more numbers. I basically have to click on the message then go up and input another number. It is quite daunting going up and down between the TO and the message.

Is there a way that I can just add more numbers without clicking on the message? I tried adding space but it just combined all the numbers together. These are not in my contact list so I can't click on +.

TIA!