I am still figuring out how to use mp3tag.

I am thinking of shifting to Roon from WMP for my music library. In WMP I rate songs and have rated ~19000 songs. Now I need to bulk tag these songs as 1-5 stars in Roon to continue the usage.

I tried using ROONTRACKTAG to add WMP Rating but I am not sure what I am doing wrong.
I added files in mp3tag. Under customized columns added a new column ROONTRACKTAG with value and field as %rating wmp%.

I don't see anything in Roon. Can anyone help me with the process. Thank you!

Given, that

is the correct field name, you have to create a field with that name - naming a column like that is not enough.
To copy one field contents to another, see the FAQs

something like this?

sorry, if my questions too dumb.

Did you have a look at the FAQs?
I think you have to copy the contents of the field %rating wmp% to the new field.
It is not enough to define just the display in the columns.

You'd also use %ROONTRACKTAG% for both Value and Field in the column definition. You can then try if it's working by entering something to that field.

In addition to the actions pointed out above, you can also just use the converter Tag - Tag using ROONTRACKTAG as field and %RATING WMP% as formatstring.