When texters send a donation the shortcode that they send the text to is 70004.
For example you may ask then to text - GIVE £5 - to 70004, or to simply text - GIVE - to 70004 where the amount will be the default value setup for the campaign - eg £3.

Where there are various amounts that could be donated to the campaign, each amount has been setup with a specific price point and associated shortcode which will then be used for the automated reply to the texter.

These shortcodes and related price points are -
70004 is the £3 / default bounceback pricepoint:
70555 - £5
70004 - £3
70444 - £1.50
70707 - £10

Other shortcodes -
70555 - if a texter wishes to skip the next transaction on a recurring gift  then the SKIP replies must be sent to this shortcode to be picked up.
70004 - is the shortcode that is also used by a Free to User campaign

See When does support end for Blackbaud Mobile Text Fundraising