Getting the username and password from mailtrap is easy but isn't super obvious (at least the first time I tried to find it).
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Click on your username and then click on the gear icon as shown in my screenshot.
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From here in the SMTP settings you will be able to get your username and password
Pretty simple, but the first time I was trying to find it, I was clicking in all the wrong areas trying to find the username and password in mailtrap.