I have a wired doorbell that I don’t want to change, but I’d like to use it as an input for SmartThings actions. I can’t seem to figure out what type of device I would use for this. Basically, the plan is to wire something into the 24v transformer so that I can get notified via ST when the doorbell is pushed. Note that I’ll likely disable the chime so putting a contact sensor on that wouldn’t be an option. I’m also not interested in removing my existing wired doorbell.
The following device is designed for exactly this purpose, and is quite popular:
There are a number of other devices and methods for doing this. See the quick browse list in the community – created wiki, look in the project report section, and look on the list for “doorbells.” 
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I cheated and used an open/closed sensor that had terminals for an external reed switch. The reed switch is triggered when the magnetic doorbell chime is actively being powered. I left the chime on, because of situations where other people might be present in the house, while i am not, and need to be alerted of someone at the door.
Which sensor did you use?
and
Here is an older thread on this topic that has some good information and pictures too.
Yes, that method and a couple of others are listed on the doorbell projects list in the community – created wiki that I linked to above. 
I’ll probably try this one. I’m in Canada so the other options are quite expensive. Thanks