The issue with using IFTTT is that it can introduce additional lag. At my house it’s about a consistent eight seconds, but some people have IFTTT lag of up to two minutes.
When we talk about lights coming on, we’re usually aiming for a response of under three seconds, and to be honest, anything more than about a second and a half gives people the sense that they are standing there in the dark waiting for the light to come on. So I just don’t know if it’s going to fit your specific needs. But it would certainly be worth trying.
I’m a little bit confused about why the motion sensor is in the picture here. Or rather why both the motion sensor and the lock are in the picture. Why not just have the light come on when the door is unlocked? Or just when motion is detected? What would be different about having all 3 devices in your rule? Or to be more specific, can you write out what your desired if/then statement would be? Nothing complicated, just “if this happens, then that should happen”.
As for why I asked if you were using iOS or android… The official SmartThings features are pretty basic rules. Essentially " if A then B." If motion is detected, turn on the light.
You can set some additional qualifiers around that like time of day or geopresence or mode, but the official features don’t generally let you have what are called stacked conditionals: “if a and B, then C” as far as devices go. So with the official features, you can’t create a rule “if the door unlocks and motion is detected in the hallway, then turn on the light.”
The good news is that there are several methods available for SmartThings which do allow you to have stacked conditionals.
if you have an iOS phone, there is a $10 third-party app called “SmartRules” which is very easy to use and allows you to have a “while” option in a rule. So “if the door unlocks while motion is detected” etc. it’s very popular and works well.
If you have an android phone, you can do something similar with a combination of two paid third-party apps, Tasker and sharpTools
So those are pretty simple add-ons.
If you want to build really complex rules, there is a free community – created rules engine called webcore. It can be quite challenging to set up although there are many community members who will be glad to help you. Usually people with a strong technical background love it and people who are looking for easy non-technical rule pbuilders will choose SmartRules instead. And some people use both, reserving webcore for the stuff that only it can do. 
Anyway, the point of all that is to start by just writing out in English what you want your rule to be in an if/then format.
If the structure is simple enough, you can likely do it with the official features like smartlighting.
If there is a “do while” component, and you can do it with SmartRules, that’s a fast and easy way to create those rules.
If you rule is really complicated and involves checking the “state” of a number of different devices as part of the if, then you may need to set up the rule in webcore.
Going back to the beginning, as long as all of the devices can be recognized by your SmartThings account, there will be one way or another to set up even extremely complex rules. But you may use different methods depending on the complexity of each individual rule. Or if you have a strong technical background you may just do everything in webcore. But a lot of people with iOS devices do as much as they can in SmartRules because it’s just a really easy app to use.
We still need to know the model of the Schlage lock, though.