If I do journal entry and choose the sales tax code. How do I get, a tax account is automatically sales tax account?

The sales account may be assigned in the tax code records. In your JE, you can only see the sales account after the saving the transaction and checking the GL impact.

Thanks, but in GL-impact, it always shows the purchases tax account?

its reading which side of the register probably. tax code GL mapping is defined on the code. You can look it up

There's a Tax Account column on JEs. The Purchase Tax Account is selected as default on JEs. If it's a sales-related transaction, you need to manually update the column value.