What you need is your username and password (the same one you use to access SFMC). You would need to ensure that your user is a "API enabled" first though.
See below from SFMC documentation:
Authenticate with UsernameToken
You can use a Marketing Cloud username and password to authenticate
your calls. UsernameToken authentication is simple but not as secure
as access token authentication.
When setting up the Marketing Cloud user, select API User. For
Marketing Cloud accounts with role-based permissions, select the Role
| Email | Admin | API Access | WebServices API permission. For
accounts with legacy permissions, select the Grant the user access to
the web services permission. Use the Marketing Cloud username and
password to authenticate your SOAP calls in the header.
Sample Header below: