Is It Possible To Bind The # Key...

...on a german keyboard? This is a seperate key, it is not on Shift+3.

Starting with the obvious- did you try # and see what happens?

Edit: digging around found these charts but not sure the relevance:
https://community.bistudio.com/wiki/DIK_KeyCodes
DIK_BACKSLASH or key 43 appears to be the key in question.

bind # input /lockon
Invaild key provided.

bind DIK_BACKSLASH input /lockon
Invaild key provided.

bind \ input /lockon
Invaild key provided.

Can see valid bind keys here. # is not a valid bind key, it is modifier like shift or ctrl.

You need to find what key mapping is the key you want to bind from the key mapping list. Then use the text description, not the code map (DIK_XX) to bind it.

Maybe keyboard layout will show you the key code, idk. If the key you want to bind is not on the key mappings list you can't bind to it.

But you can use remapping software to change the german # key to something windower knows about, eg one of the special keys at the bottom, and use that.