Not sure that I understad your question.

In addition to the below, you can create a page with the following shortcode:

[frontier-post frontier_custom_tax="category" frontier_custom_tax_layout="readonly"]

and use this link: https://yoursite.com/your-page/?task=new&frontier_cat_id=7

If you place the widget “Frontier Add Category Post” in a sidebar or header, it will only show on category pages, and offers a direct link to create a post

I answered the below, on your other post, this is also relevant on this:

One thing to be aware of is that you need to use double quotes for short code parameters

you can use this shortcode:

[frontier-post frontier_mode="add" frontier_cat_id="7" frontier_list_cat_id="7" frontier_custom_tax="category" frontier_custom_tax_layout="readonly"]

Where 7 is the ID of the category (you can add ID to category listing in the admin panel in Frontier advanced settings). You can add several category IDs separated by commas – ex:frontier_cat_id=”7″

frontier_mode=”add” will go directly to the entry form

More info on wprfrontier.com

https://wpfrontier.com/frontier-post-shortcodes/
https://wpfrontier.com/frontier-post-layout/
https://wpfrontier.com/frontier-post-category-posts/
https://wpfrontier.com/frontier-post-add-explanation-to-taxonomy-section-on-form/