Expression cannot begin with "$\sum_{j=0}^{\floor*\frac{a}{b}} j$"

Question

This is my first time using Wolfram Mathematica and writing in TeX. I'm on the free trial version of Mathematica, and I'm writing in a notebook, using the "Inline TeX Input" option on the "Insert" tab. Previously, I have only ever written mathematics in MathJax.

I tried the above first without the dollar signs.

It didn't work then either. I figured maybe the problem was the lack of dollar signs. It appears not. A further confusing detail here is that the slash in front of the sum is red in the last picture, whereas it isn't in the one above. So I thought, maybe I have to add a dollar sign in front of all of them (not that that makes any sense to me, but I have no idea what I'm doing).

It still didn't work, but in this instance, the slashes aren't red anymore. I don't understand why this isn't working, according to this guide, sigma functions are displayed by writing "\sum".

EDIT:

I tried doing as instructed in the answer by @Nasser, but upon pressing enter, the box turned red. Here's the code:

 Failure["TeXAssistantError", Association[
   "MessageTemplate" -> "Errors: `Errors`", 
   "MessageParameters" -> Association["Errors" -> {}], 
   "Input" -> "crtl$ \\sum_{j=0}^{3} j"]]

Answer 1

You have to first type ctrl$ then type the LATEX there with no $'s, since it is implied math mode. like this

Which was entered using

Then hit the return key.

For more info, see InlineTeXInput.html

btw, no one really writes * in Latex as multiplication is implied with just the space. Better not use * in Latex.

Here is a video. Hitting CTRL$ is just a short cut for using the menu. Both work the same.