Can't sneak with my model

hello to the whole community.
I start from the premise that I don't even have the programming basics.
I'm working on MCreator to create a mod that allows me to upload race models I have created.

so far I have been able to load the test model and apply it to the player via an external mod.

and now comes my problem in which I request the support of the community .... once the model is applied to the player, by pressing the shift key the character does not crouch like the basic player model could someone direct me or help me in solving the problem?

thank you in advance for your participation.