Is it Necessary to Specify the Intention for Voluntary Prayers?

Specific voluntary prayers like Al-Witr, two rak'ahs of Ad-Duha, Ar-Rawatib of the five obligatory prayers, these are specific prayers, so you must specify (the intention) in them. You must specify them with their names. However, it should be in the heart and not uttered by the tongue. For example, if you wish to pray Al-Witr and you made the Takbir, but you did not have the intention of (praying) Witr at that time, and then during the Salah, you intended it to be Witr. This is invalid. (That's) because Witr is a voluntary prayer, and voluntary prayers must be specified in itself. General (non-specific) voluntary prayers do not require a (specific) intention except for the intention of prayer. (Having the) intention of prayer is a must. For example, in the morning/forenoon, if a person performs ablution and wishes to pray whatever Allah wills (for him to pray); we say, it is enough for him to just have the intention of prayer. That is because his prayer is not specific.