Mathematica aside

I have used a mix of input notation to show how it can be done.

I would enter the dynamic boundary condition from a key pad as

In Standard form, which is a little shorter, you would need the type set window to make this preactical.  

Finally it may be more easily read in Traditional form as

Note that you can convert any "cryptic" input expression by selecting the cell, going up to the cell menu and selecting "convert to traditional form".  This is "shift curly t" on the keypad.