I would encourage to use password type of property for password & string for username as suggested by Ashutosh.
But still if you want to set single property then you would require to use json string as value; for better maintenance
something like this
Name - authenticataion_details
Value - {"user_name":"abel.tuter","password":"abel123"}
Then you will have to parse it where you want to retrieve it
something like this
var val = gs.getProperty('authenticataion_details');
var parser = JSON.parse(val);
var userName = parser.user_name;
var password = parser.password;
Regards
Ankur
