How do I add a ~ to a spanish word via text message on my iphone 6S? Example: manana?

How do I add a ~ to a spanish word via text message on my iphone 6S? Example: manana? - 1

Press and hold the letter you want to have an accent. I assume you mean like this: ñ?

Hope that answered your question.

Most keyboards display letters with diacritical marks, like tildes "~", by holding down the letter key: ñ.