The Safari actions require Safari to be open to the page you want when you launch the activity. That said, you can accomplish this easily using another element.
I don’t know your technical level, so I apologize if any of this is stuff you already know, but for you and for future reference to anyone looking, here is the howto.
First, go to the page you want on a Mac/PC browser and open the developer tools. I use Firefox Developer Edition, but Safari and Chrome have them also. If you’re using MacOS Safari, you will first need to open Safari’s Preferences menu and check “developer options” and relaunch.
Once the developer tools are open, switch to the Network tab / pane; find and turn off the option which clears the log on each page load. Keep this pane open, fill out the form, hit the trash can to clear the network log, and then submit the form. Now the HTTP request of the form submission should be either the first or the only thing in the list.
Select the line for the form submission and look at the request header. There should be a Request Method, Request Url, and all the key-value pairs that were submitted with the form.
Back in the Shortcuts app, add a URL element and give it the same URL as the Request Url. Chain this URL input element to a Get Contents of URL action element and toggle open the Advanced drawer. If there are any additional headers in the request log, you can tap Add new header to add more. Match the Request-Type to the Method field, choose “Form” as the Request Body, and then add in all the keys and values from the request.
Anyways, that’s it.
Chain this to Set Variable then use an IF block to test if the login succeeded or not. If it succeeded, make a Show Result that says “You are now logged in.” and if it failed, show the variable so that you can debug the response.
Share Sheet
section at the top level of "My Shortcuts" – Cameliacamella