One thing you can do is the following. Start by entering a derivative in a cell with StandardForm, for example:

D[f[x], x]

or

Dt[f[x], {x, 2}]

You can then select the cell (via the bracket at the right of the window) and either

1. Type CMD+Shift+T (or CTRL+Shift+T on Windows), or
2. select from the menu Cell > Convert To > TraditionalForm

You should now have an Input cell which has been converted to TraditionalForm; for the examples above this results in

or

At this point you can set your cursor where the f(x) is and replace it with any expression you prefer. Note that you may need to use grouping parentheses ( ) to ensure the correct derivative is taken. For example,

or

The above forms aren't the most attractive. Interestingly, however, if you again select the cell, which is now an Input cell in TraditionalForm, you can switch back to StandardForm and again back to TraditionalForm (see 1 or 2 above) and it 'cleans' up the display; in this case it removes the parentheses but still give the correct derivative as Output.

Of course, you can input the expression for which you want the derivative in StandardForm when you begin, but it doesn't sound like that's what you're after.

One thing you can do is the following. Start by entering a derivative in a cell with StandardForm, for example:

D[f[x], x]

or

Dt[f[x], {x, 2}]

You can then select the cell (via the bracket at the right of the window) and either

1. Type CMD+Shift+T (or Ctrl+Shift+T on Windows), or
2. select from the menu Cell > Convert To > TraditionalForm

You should now have an Input cell which has been converted to TraditionalForm; for the examples above this results in

or

At this point you can set your cursor where the f(x) is and replace it with any expression you prefer. Note that you may need to use grouping parentheses ( ) to ensure the correct derivative is taken. For example,

or

The above forms aren't the most attractive. Interestingly, however, if you again select the cell, which is now an Input cell in TraditionalForm, you can switch back to StandardForm and again back to TraditionalForm (see 1 or 2 above) and it 'cleans' up the display; in this case it removes the parentheses but still give the correct derivative as Output.

Of course, you can input the expression for which you want the derivative in StandardForm when you begin, but it doesn't sound like that's what you're after.