Most units are designed for user to add or qualified AC person. If you add a can you most likely will be up and running. These units tend to leak more in the off season when the compressor hasn't self lubricated it's shaft seal in months. Some units have been fine after adding and running them frequently.
If it leaks out fairly soon, then you have spent $5.00 to find out that you may need the $75 an hour mechanic. Do the logical method first unless you have $$ to burn. If yours leaked it all, it wouldn't be running at all now.
Brian