The Via field should show on the defailt log form. Just type BURU in it. It is not a user-defined field.
It doesn't have to show on the report to accomplish what you want. You just want to use it to select off of.
Run the QSLing report of choice, click the Filter button marked with the red 1 in my screen shot. Fill in QSL SENT and VIA as shown (2). Click the OK button.
This example is for printing buru cards. To print other cards, not buru, do the same thing except select <> NOT EQUAL where the = is to the left of Via in area 2 of my screen shot.
Let me know if any questions.
73,
Dennis