Assuming your Month Year dim object is in MM/yyyy format.
Drag your Month-Year Object in your WebI Query Filter and make it as prompt, You can enter the text for prompt as "Enter Month/Year". This will be your P1 prompt which takes the month year value from user.
Then in your WebI create a Variable and use the below formula. This is your P2
=SUBSTR(USERRESPONSE("Enter Month/Year" );LENGTH(USERRESPONSE("Enter Month/Year" ))-4;4)
This will extract the year from the prompt. You can concat it with the fixed month