What is the formula to continually add a number inside a single cell with
itself over and over and total shows up in another cell? Almost like a
running total from 2nd cell from input in 1st cell.

i.e. Cell B2 has an a user input of 1, cell D2 totals 1.
Cell B2 has a user input of 2, cell D2 totals 3
Cell B2 has a user input of 5, cell D2 totals 8, etc.