The thing is that while it is not semantically an order it essentially works out to be an order.
Say you meet your boss a few days before your yearly review and he says "I would really like if this project is finished TONIGHT!"
Now, did he say "I as your boss command you to finish tonight!" no Did he say "Finish the project tonight or you are fired!" no
But he did strongly tell you that his pleasure is that it is finished tonight and you understand that you will be reviewed as an employee in a few days, so you will likely want to do that to please him to get a good review. Yes he never said you needed to do it to get a good review, but common sense says that if it is important enough for him to say he really wants it, not doing it means you get a bad review.
So Trump saying this to Comey was essentially saying "if you don't do this, I will probably fire you" which is very close to illegal if not totally illegal
I agree. Leaning and leverage to lay off Michael Flynn, in the role of President.