There are many references in pop culture where they will present a crusader or templar like character, with the caption Deus Vult. I'm trying to find their counter part, that being the Seljuk warrior, (I am it sure of the correct military rank or term for a Seljuk "Knight") and what their war phrase would be.
No. The Deus Vult meme that you're familiar with in popular culture is ultimately a reference to a distinct event in the history of the Crusades: Pope Urban II's address to the Council of Clermont in which his claim that God wills the recapture of Jerusalem was supposedly met with spontaneous cries of "Deus vult!" or "Deus lo vult!"-- God wills it!
Now, some of our European medievalists might be able to address how widely this phrased was used beyond this incident, or if this incident even is likely to have happened as I am describing.
But there is no corresponding moment or phrase on the Islamic side.
"Allahu Akbar", often translated as "God is great" is actually somewhat grammatically confusing because in Arabic it's either a comparative or superlative, i.e. "God is greater [than anything else but especially what one is immediately facing]" or "God is greatest". There is a set phrase for its pronouncement, "takbir", that is "the saying of 'Allahu Akbar'" which I will now use to describe it.
Unlike "deus vult", the takbir is central to the practice of Islam. To give two short examples, it is Quranic, and in the five-times daily calls to prayer it is said six times altogether. I would encourage you to hear what this sounds like, for example, in contemporary Cairo: https://www.youtube.com/watch?v=EFlWJj0pv-Q& So, excluding all other uses, this is a phrase that has been said to call people to prayer 30 times per day in every Islamic city in the world every day for the past 1400 some-odd years.
While it can and historically has been a battle cry it also performs other functions, including as a cry of mourning. For example, when the ruler of Abyssinia, whom Muhammad described as a brother, died: "the Messenger of Allah announced the death of An-Najashi to the people, and he led thme out and arranged them in rows, and said the Takbir four times."
I don't know enough about the Turkic languages of the Seljuks to tell you what their particular battle cries would have been. They might well have included "Allahu Akbar" which as I mentioned is a perfectly valid battle cry. But it is not the equivalent of deus vult and has a much deeper and more profound meaning within Islam.