If I order a coffee and want to take it out. (To go..) The expression "I'll take it as take out." makes sense? And use that in common?
The repetition of "take" is awkward, so your example is not the best choice.
When it comes to ordering food, sometimes you can just tack on a short phrase which is not a full sentence:
"for take-away" chiefly British
Adding the word "please" at the end is a good idea.
Of course, full sentences are also possible:
"I'd like it to go please."
"I'll have it to go."
"I'll have it as take-out."
"I'll take it to go."