How can I bind 2 keys (key + mouse button) on one action?

Hello Everyone, I wonder how can i now bind 2 keys combination to one action? The issue i have right now is that i wanted to change from Shift+MB2 to Space+MB2 for autoattack move. Unfortunatelly now it is only Space. and i can't make it Space+MB2. Can someone help me how to do this?
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