Configure STM32WB55RG for Mbed CLI2

Hi,
I am using Mbed CLI2 to add a custom target STM32WB55RGV6. My system is Windows10 and I am using latest Mbed version. Since Mbed CLI2 supports boards K64F, DISCO_L475VG_IOT01A and NRF52840_DK only, I tried to add a custom target for my requirement following the given link.
https://os.mbed.com/docs/mbed-os/v6.6/program-setup/adding-and-configuring-targets.html
When I tried to configure the new target, error is obtained that 'Attributes are not found'. Is this the right step to be followed or is there some other steps to be followed?
Also since the version of Mbed CLI2 is beta, is there any other option to add a custom board?

Thanks & Regards
Keerthi

Start with the Mbed OS v6.6 “Adding and configuring targets” guide linked in the issue, then reproduce the custom STM32WB55RGV6 configuration with Mbed CLI2 on Windows 10. Done means determining whether the documented process supports this target and resolving or clearly documenting the “Attributes are not found” error.

Written by the indexing model from the issue text.