I can confirm that the way Scripted Fields are designed in ScriptRunner for Jira Cloud means that you must enter an issue Key in the Test Against Issue box and click the Test button to test the script first.
This is explained in more detailed in the documentation page here which lists the required steps to create a Scripted Field.
Once the script has been tested and has a valid return type then the Save button will become visible and available to use.
I hope this information helps.
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