First gain access to your email address that you used to sign up for textnow. Most email web sites have a menu choice for forgot password.
If that is not possible for some reason, unfortunately, you won't have much luck, as the forgot password menu for textnow uses that email address.
Once that is fixed, the next step is the forgot password for textnow menu.
https://www.textnow.com/forgot
https://www.textnow.com/forgot
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