How to enter ANY amount vs. a fixed amount

For integration of Authorize.net - SIM

[ccasati]

We are following the SIMs integration for authorize.net but we need our clients to be able to enter ANY amount into the amount field, not just a fixed amount.

Any help on this is greatly appreciated, it would be much more helpful than any of the authorize.net manuals!
(You would think this is a common question!)

[rajivlodha]

Quite confusing, I guess you are asking for donations etc?

Create a textbox where user can enter the amount he wants to pay, using some server side script, grab the entered amount and attach the same with SIM code (http://developer.authorize.net/samplecode/)