Field/column of a database table

How can I select a single column from a table, when using expressions?

{{table}} is the only thing I can select in expressions, but I want something like {{table.userid}} - userid being a column in that table.

Because I want to do a API action with that afterwards, but now my API call is something like
/?mrp%5B0%5D%5Buserid%5D=7867

But i need it to be like
/?userid=7867

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