Mathematica aside

I have used a mix of input notation to show how it can be done.   Also, some of the major equations are shown in input form and also in typeset form for clarity.  The typeset input equations are usually marked as inactive and so are not read in by Mathematica .. but they could be.  

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

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

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.