How to set a free shipping discount code for one option only?

[EdithCC]

Hi all,

I am trying to set a free shipping disount code. I only want to offer free standard shipping. I have tried to test the code, but I found that all shipping options become free including xpresspost. How can I set a discount code which offer free shipping on one option only while other options like xpresspost will still charge? Thank you so much!

[TC13]

Would really love this issue to get straightened out, any luck?