In my work as a developer, I need username/password combo's from the customer sometimes to make sure the settings on 3rd party services are correct to work with the website/application we are building. E.g. a payment provider that we use on a game website.
What would be the good way to ask for their username and password so that it stays secure? If I send them an email, for sure, they are just going to send me back a plain text email with the password, but that is not secure.
I see my question got a bit misunderstood, so I will add an example:
Suppose I am a developer for CoolSoft, a software company. Another company (let us call them Games, Inc.) wants us to create a website for them. This website will use 3rd party services for payment and customer support which we need to integrate with. Games, Inc. creates accounts with the payment provider and the customer support provider. But they are totally not technical and we need to have their credentials to set the correct callback URLs, etc.. How can they safely send their password to us so we can fix the settings on their accounts (We never meet in person)?