Here is a complete example, starting with a worksheet like this:

Click on the specific cell containing the value with which you want to populate a new column (B2), enter a name in the name box (cellCategory), and press Enter. Right-click the cell and click on Get Data from Table/Range... which opens the Power Query Editor.

Open the Advanced Editor, delete everything, enter this line of code and click on Done:
Excel.CurrentWorkbook(){[Name="cellCategory"]}[Content][Column1]{0}

Named ranges that are loaded in Power Query are automatically transformed to a table. In the line above, [Column1]{0}
accesses the value located in Column1 at row index 0 of that table which contains a single cell. So now you have a query that returns the content of that cell.
If your main table isn't yet loaded to Power Query, return to the worksheet, right-click on your table and click on Get Data from Table/Range....
Now, as shown in Step 5 of the linked tutorial, go to the ribbon tab Add Column, click on the button Custom Column, and add a new column named Category filled with the cellCategory query value by entering cellCategory
and clicking on OK:

Here is the result:
