The public key is not available

I am trying to setup a gateway server and i have 2 errors:
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY D8FF8E1F7DF8B07E
for: InfluxData - Package Repository stable InRelease
InfluxData - Package Repository bulleseye InRelease

What can i do? Can someone tell me step by step how to solve this please?
Thank you!