auto save in iCloud email on my laptop

Open mail application , use command and N keys to compose a mail Keyboard shortcuts in Mail on Mac - Apple Support , fill in the To , From field , write some sentence it will start saving in drafts , click on red dot in left corner of the window , a pop up will appear , click on don't save .

You must never leave compose mail window before quitting mail application if you are not sending mail to recipient ( that the reason use command and W keys ) to close it , again right click on mail app and click on quit .

When mail is composed and you write some sentences in subject field , it will start saving in drafts you can close the window ( use command and W keys ) and click on don't save .

For web gmail account see the article https://support.google.com/a/users/answer/9308990?hl=en