PreFill From an external datasource

I want some of the fields on my form to be populated automatically like we have in Nintex K2 when I populate a particular field.

I.e when I populate a particular field, I want other fields on the form to be filled automatically by getting their data from an external datasource like sharepoint or sql server.

Filling all the form manually consumes a lot of time and is stressful hence, the reason for asking how to implement this feature