OPNsense as a L2TP client - no pre-shared key authentication option + troubles

[pfabi]

Hi Guys! I looked for tutorials and forum posts that might be helpful but to be honest, I couldn't find the solution. I have UDM Pro sitting in on of my sites. It acts as a L2TP VPN server. No issues with connecting other devices. I wan't to connect OPNsense to it, as a client (only way to establish VPN connection to this site without using jump stations etc.). My issue (at least that's what I am thinking) is that there is no pre-shared key/"shared secret" field in the OPNsense configuration that could be filled. There is username and password, but no "shared secret" that is required in this case. What am I missing? Maybe there is some documentation that could help? Thank you in advance!