You can do that during the process of recording a new macro.
Tools | Macro | Record New Macro
In the dialog that appears you can assign a name to the macro and choose
what key to press in combination with the [Ctrl] key to quickly run that
macro in the future.
Once you've done that, decide if you want this to be available in the
workbook you are working in only, or if you want it available all the time.
Click OK and perform the operation: simply type the value into a cell and
press the [Enter] key, then click the [Stop Recording] button (or if you
don't see that, use Tools | Macro | Stop Recording).
You'll probably need to modify the code generated a little bit because when
you pressed the [Enter] key during the recording (you had to, trust me) the
macro recorded which cell was selected after entering the value and that
became part of the macro. Easy way to edit it:
Tools | Macro | Macros
Highlight the macro you recorded and click the [Edit] button.
In the code you'll probably see something like:
ActiveCell.FormulaR1C1 = 377.88998774
Range("A2").Select
Simply delete the Range("A2").Select line of the code and close the VB
Editor. Now it will work to place the value in what ever cell you have
selected when you call the macro without always jumping back to cell "A2" (or
whatever cell reference was in the macro you recorded).