how to add text valuecolumn in data sets

Hello, Example of current situation: I have 2 separate data sources that I am collating into the same table using UNION DAX command, However, each table does not have any sort of unique identifier. Therefore, I would like to add a new column to each table with a unique identifier such as 'TABLE1' and 'TABLE2' So that when the new table is created using UNION, I will be able to see which row is coming from which table/source (Also allowing me to create my reports based on the unique identifiers). So what I am simply after, is a way to add a new column to an existing table which I can specify a text string that will input into every row.

Thanks in advance.