Headset microphone is not working in ubuntu 20.04

I also recently changed from Ubuntu 18 to Ubuntu 20.04 LTS and had a similar problem, just that I use headset with a 3.5mm jack.

I resolved it by:

1. Searching https://www.kernel.org/doc/html/latest/sound/hd-audio/models.html for my laptop model Acer Aspire, I got aspire-headset-mic (Headset pin fixup for Acer Aspire);
2. Opening /etc/modprobe.d/alsa-base.conf file in VS code;
3. Adding options snd-hda-intel position fix=1 (on line 44) and options snd-hda-intel model=aspire-headset-mic (on line 45);
4. Saving the file, restarting the laptop.

Next time I plugged-in the headset, I got the window asking me Select Audio Device (Headphones / Headset).

*Found that link on the second answer from Headset microphone not working on Ubuntu 20.04.

**The video that you posted here also helped me, together with the 1st comment (from user Insp3ctorJon3s) on the video.

Not enough reputation to comment, therefore new answer to @xaif.

My workstation (Lenovo Thinkpad P53) is not on the list either. Nonetheless, I checked my audio codec with the command cat /proc/asound/card*/codec* | grep Codec, then looked for the Conexant CX8070 in kernel.org. The specific model was not available but the generic headphone-mic-pin added to /etc/modprobe.d/alsa-base.conf did the magic.

Summarizing:

1 - type cat /proc/asound/card*/codec* | grep Codec in terminal;
2 - copy audio codec (mine is Conexant CX8070) and search it in kernel.org;
3 - if the model is not available, read descriptions and see whether a generic for that producer is available. In my case, the description for headphone-mic-pin is 'Enable headphone mic NID 0x18 without detection';
4 - go back to terminal and type sudo nano /etc/modprobe.d/alsa-base.conf and add the line options snd-hda-intel model=headphone-mic-pin, while replacing the codec with your own;
5 - save and close /etc/modprobe.d/alsa-base.conf in nano (Ctrl+X quits the editor and you press Y to confirm that you want to save);
5 - reboot;
6 - plug in the headset after rebooting and you should be able to select it as audio input in Settings/Sound.

This worked for me, hope it will help others too.