in the addnews.tpl file

1. we remove the standard output list of categories, the category tag
2. define the group [group=x] text [/group] where x is the group id
3. Output the categories of the categories that are needed, where x is the category id

<select name="catlist[]" id="category" onchange="onCategoryChange(this)">
        <option selected value="x">category name</option>
</select>

4. There is 1 category in the list, and it is already selected by default
5. Using css, you can hide it, for example:

<select style="display: none;" name="catlist[]" id="category">
<option selected="x">category name</option>
</select>

6. at the output we get: publication in the category selected by default

P.S. in my opinion, a very simple solution (so you don't need to create a bunch of templates to add news, no edits in the engine)

Thanks for your attention!