You don't need an API key to use the BrandMeister Network. You probably mean the server password, which is the "hotspot security password". Please take a look at this post: https://news.brandmeister.network/configure-a-security-password-for-your-hotspots/
You need to check the hotspot security setting on your BrandMeister SelfCare page at https://brandmeister.network/
If hotspot security is turned on, you have to use the hotspot password you've set there as the server password in the openSPOT. If hotspot security is turned off, then you have to use the default server password which is "passw0rd" (without the double quotation marks). Note that now all servers require a hotspot security password to be set and enabled.
Also read this for more help on connecting to BM servers: https://help.brandmeister.us/hotspots/common-connection-issues
After your openSPOT4 is connected to the BrandMeister Network, then you can use the BrandMeister Manager to link/unlink talkgroups through the OS4 web interface. The BM API key is required to use the BrandMeister Manager.