You have to do a create action. Then you can use the output of this action in the Call REST action.
Still plenty to do:
- define your data model with an entity LoginAttempt having attributes Name and Password
- create a export-mapping
- call the REST-service using the exportmapping
- import the result, probably into a variable ‘CredentialsAreValid‘ of type boolean
This will help you out: https://docs.mendix.com/howto7/integration/consume-a-rest-service#1-introduction
Microflow is as follow