I created an account for my client. I can login into the website fine with credentials. I downloaded Dynu IP Update Client v4.43 and installed. On the Activity tab, entered the same username & password, Save, shows:
11:12:41 AM>IP address update aborted due to invalid password.
11:12:41 AM>IP address update initiated.

11:11:41 AM>[badauth] Username AndresAvelar9890 or password is invalid! Please check the settings and try again.
11:11:41 AM>Server response: badauth
11:11:41 AM>Detecting IP address from server side.
11:11:41 AM>Updating IP address for username AndresAvelar9890.

You do not have a hostname in your account. You may sign up for a hostname at https://www.dynu.com/en-US/ControlPanel/AddDDNS and the IP update client will update the IP address of that hostname.

I thought I already set it up as: TerraScape.dynu.net
When I click on your link and re-enter, and click Add, it shows: "Oh Snap. Domain name taken, try again"
how do I link that existing hostname, to my account?

We added terrascape.dynu.net to your current account. You had this hostname in another account that was not confirmed.

Thanks...
now, when I enter the credentials in the: Dynu IP Update Client v4.43, it shows the IP address...