As per Wikipedia:
Plagiarism is the fraudulent representation of another person's ... as one's own original work.
So no, "assessing the similarity index of your manuscript" is most certainly not plagiarism.
However, according to https://en.wikipedia.org/wiki/Turnitin:
The essays submitted by students are stored in a database used to check for plagiarism
I'm not sure what exactly Turnitin does with the stored materials, e.g. who can access them. Since your work will be stored there, it can potentially violate the copyright required by your conference. I would check with the conference whether using Turnitin is allowed (and, as others already mentioned, I wouldn't use any such tool in the first place).