Here is what I could dig up(the last 2 were both from tls mode options):

'OpenVPN's usage of HMAC is to first encrypt a packet, then HMAC the resulting ciphertext.'

'--tls-auth file [direction]
Add an additional layer of HMAC authentication on top of the TLS control channel to protect against DoS attacks.

In a nutshell, --tls-auth enables a kind of "HMAC firewall" on OpenVPN's TCP/UDP port, where TLS control channel packets bearing an incorrect HMAC signature can be dropped immediately without response.

file (required) is a key file which can be in one of two formats:

(1) An OpenVPN static key file generated by --genkey (required if direction parameter is used).

(2) A freeform passphrase file. In this case the HMAC key will be derived by taking a secure hash of this file, similar to the md5sum(1) or sha1sum(1) commands.'

'It should be emphasized that this feature is optional and that the passphrase/key file used with --tls-auth gives a peer nothing more than the power to initiate a TLS handshake. It is not used to encrypt or authenticate any tunnel data.'

According to this the HMAC key is static and is not used in the authenication.(2048bit FYI)