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There are plenty of ways to do that.

Option A. From the Category itself.

Create a menu item of type Akeeba Ticket System, Category (or Akeeba Ticket System, Categories).

This presents existing tickets in the category, along with a New Ticket button which creates a new ticket into that category; no selection necessary.

Option B. With a menu item.

You can create a menu item of type Akeeba Ticket System, New Ticket.

When you do that, there is a Category option in the Details tab. Click on Select next to it to select your (sole) category.

Using this menu item will always create a ticket in this category.

What you can and cannot do

You cannot completely remove the Category field from the ticket form. This is not a limitation of Akeeba Ticket System; it is a limitation of Joomla's Fields feature (a.k.a. custom fields). For them to display on the page there must be a Category field in the form so that Joomla! knows which fields to display. Here's the kicker. Joomla cannot know if there are any fields to display unless there is a Category field. Therefore, we cannot remove the Category field because the condition for removing it depends on the core Joomla Fields feature which depends on the Category field's presence. It's a chicken and egg problem.

Yes. Use Option B described above. Every Joomla menu item, even in a menu category which does NOT have a corresponding menu module, creates a unique URL in Joomla. Therefore, you can do a template override for the ATS module to link to this specific URL instead of the generic New Ticket page.

Did you follow the documentation directions on how to create a hierarchy of menu items for ATS?

Oops! That would do it :) I am glad it's all working now. Have a fantastic day!