How to save data from field

I'm new using pega system and still learning about it. I have a trouble about saving the data from field to data page. What step must i do and can give me some explain ?

I'm using pega 8.1.1

Here is my image :

The Data Page: https://ibb.co/tZYZ0qh
The Login Form : https://ibb.co/ZYRdY6x

In the past i was trying to give the target but it doesn't work My Target :

D_LoginCredentials[Password:.Login_Credentials.Password].Password

Here is the image : Target Data Page : https://ibb.co/h83mtc3

I want to save the username and password after submit so i can do authentication for user, if user was submit it do checking the password that was right or not, if new user i want to add the data to page data.

Thank You

No need for the Data Transform if you just bind the controls in the Section directly to the properties on the Data Page.