As @Gutheinz mentioned, you will need two devices.
First you need something to control the garage door opener itself, again, as he mentioned, people usually use the Nortek garage door opener which costs around $80 and is sold under many different brand names including Nutone, Linear, Iris, and GoControl. It’s on the official “works with SmartThings” list. 
https://www.amazon.com/GoControl-Linear-GD00Z-4-Z-Wave-Controller/dp/B00M75TEIU
Next you need a keypad device. There are two that work well with SmartThings and both are popular in the community. They have somewhat different form factors and so different people will like different ones. Neither is 100% weatherproof, so you will probably want to put them in a plastic enclosure box.
The first is the Centralite keypad. It is sold under both the iris and Xfinity brand names. For some reason it gets terrible reviews at Lowe’s, I think people must not know how to set it up. But it’s a pretty good device and is quite popular in this community. It’s often on sale at Lowe’s for around $25 if you want to wait for a sale.
https://www.amazon.com/Iris-Smart-Home-Management-3405-L1/dp/B01GCKAFX8/
You will need custom code for it:
The other option is the BENext mini keypad which is sold in the US under the Zipato name. It also works with RFID tags, which some people like, especially for kids or dog walkers. The low reviews seem to be from people who didn’t understand how it worked, it has gotten very good reviews for the European version.
https://www.amazon.com/Zipato-wt-rfid-us-Keypad-Z-Wave-Reader/dp/B00KMU55TY/
Again, it does need custom code:
Both allow for multiple user codes, when used with SmartThings both will allow for schedules, such as only allowing the dog walker’s code to work at a certain time of day, both have a number of other features as well.
If you have any follow-up questions on either of the keypad devices, ask them in the thread with custom code, that’s where you’ll find people who are actually using those devices with SmartThings.