in your form, in "admin" tab and "site" tab:
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make a new field with "+"
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call it "my category" for exemple
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at the bottom of the field, set the storage to
standart | article | catid
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Save and close it
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click on 2 (on the right of your added field)
on the line of your "my category" field, you can write the ID of the category where you want your next/new articles to be saved.
You will find the categories ID value in the joomla category menu (where you can add new category).
If you write 10, you content will be saved to the category with the id = 10.
Got it ?
Another way:
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make a new field with "+"
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call it "my category" for exemple
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Choose Type: "dynamic"
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Table: #__categories
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Option name: title
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Option value: id
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Where: published = 1
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at the bottom of the field, set the storage to
standart | article | catid
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Save and close it
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Put it in "admin" and "site" view.
It will ask you to choose the category from the list when you will start a new article.
PS: maybe you would start reading the documentation... and understand joomla a bit more. whatever. Peace