The answer types for questions in Kinetic Request do not include a question type of password. However, you can turn any text question into one by adding a render attribute of name 'type' with a value of 'password'. Note that this changes the way the field renders, but not how it is saved or retrieved.
Simply drag over a text question into the desired location:
Name the question whatever you desire. It doesn't actually have to be named anything to do with password, we just did for the example out of a lack of creativity.
Then scroll down to the render attributes and then enter one with a name of 'type' and a value of 'password'.
Be sure to click the Add Attribute button so that it appears as an Added Attribute.
Save your service item.
Now when you view/preview your service item, and you type into that field, it will appear as a password field: