Try clicking on the edit button on the right side. That should open a window where you can use your constants as input.
Based on your error, you have a character set mismatch (UTF-8). Double check your Target URL. See if this helps "https://www.sdn.sap.com/irj/servlet/prt/portal/prtroot/docs/library/uuid/40728f7b-0401-0010-d9bc-8c73884a3789"
Create two constants of type String in module <Module>:
1: Username (default value: <Username>) => @<Module>.Username
2: Password (default value: <Password>) => @<Module>.Password
Link the constants to the fields Username and Password by pressing the button Edit…
Add the constant. For example: @<Module>.Username