Web API to Update Data of DB

Hi,
I have a requirement where i need to Update the data of the Database using Web-API, for that i created a Web API using POST method having a!writeToDataStoreEntity() function configured.

Problem Statement:- when i pass the JSON Format data into body field of Web API, it works Well, but whenever i am trying to do the same using an Expression rule where i have defined a rule input of type text where specified value is the same JSON which i used earlier, then nothing gets Update in database and also no log informations about it, and it returns Writer Type followed by following Value
[name=httpwriter_appian_internal, parameters=http://localhost:port/suite/webapi/storeEmployee; POST; ; ; [username=[externalSystemKey=mycredentials, fieldKey=username, usePerUser=false], password=[externalSystemKey=mycredentials, fieldKey=password, usePerUser=false], preemptive=true];followed by Data
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OriginalPostID-244534