In order to enable mic on a bluetooth headphones, you need to change device configuration to Headset Head Unit in Settings > Sound menu. However, if you do so, the audio quality of the headphones will be quite bad.

Pipewire introduced support for the mSBC codec. This codec allows your headphones to enable microphone and still have okay-ish audio quality.

mSBC codec is disabled by default. To enable it, create ~/.config/pipewire/media-session.d/bluez-monitor.conf file with the following content:

properties = {
    bluez5.msbc-support = true
}

Restart Pipewire:

systemctl --user restart pipewire.service

HSP/HFP, codec mSBC option should be available in Settings > Sound menu