In the Policy Manager, select Tasks > Certificates, Keys and Secrets > Manage Stored Passwords. In the pop-up box that appear, select Add and complete the form to create the stored password.

If you plan to get the password using a variable, you will need to checkmark Permit use via context variable reference

In this example, a stored password named foo was created.

You would then add the Set Context Variable assertion to your service and use ${secpass.foo.plaintext} to get the password associated with the foo stored password. In this example, the variable is named password.

Now, you can use the ${password} variable in another assertion, and the ${password} variable will contain the foo stored password.