Say, I enter a invalid value for the repo URL, for e.g "arithmetic". It errors when I click Generate Workflow. Now, if I fix it to a valid URL, I still need to click on the Generate Workflow button to see if it's valid and I will see the error from the previous attempt (Please enter a valid username/repository or URL) until then.

I think it's fine but it would be really nice if the value is auto-validated as soon as I click out of the edit box without having to click Generate Workflow again.

A similar model to when you try for a username in a gmail account. If something is already taken it error but when you type something that is possible, it gives immediate feedback that it's OK without any clicks.