Businesses must register on the site to use it.
"Any reporting organisation yet to register for the new SAR Portal will be encouraged to do so and begin using the Portal to submit Suspicious Activity Reports," says the FIU which is part of the National Crime Agency. So it's not compulsory - yet. "Please note, once [an organisation has registered], it is expected that the new SAR Portal will be the sole reporting route for all registered organisations, as the current SARs Online System is planned to be decommissioned later this year." (sic).
The announcement goes on to say "To prepare your organisation to register for and use the new SAR Portal, please review our supporting materials and user guides, which can be found on the NCA Website or the Portal Guidance Signpost."
We have no idea what a "Portal Guidance Signpost" is or why it needs capital letters.
FCA materials are here:
and here: https://www.nationalcrimeagency.gov.uk/who-we-are/publications/656-new-… (oh, that's what the signpost is).