How to get a wallet address and set label via RPC?
You may have already read this, but make sure you read this if you want to know about how the accounts feature of the core client works: https://en.bitcoin.it/wiki/Accounts_explained. (It's somewhat of a legacy feature.)

Are you looking to be able to do this manually with the RPC methods, or automated? 

For manually, do:

bitcoin-cli getaddressesbyaccount "donations"


If the JSON array returned is empty, then no address is associated. To create a new bitcoin address with that label, do:

bitcoin-cli getnewaddress "donations"


To get an address for the "donations" account, do:

bitcoin-cli getaccountaddress "donations"