Football Manager 2020

Edit: So, there is a workaround to unlock this.

Remove all your save files from the games folder. (Sports Interactive > Football Manager 2020 > Gamepass > Games). Move them to a new folder on the desktop. This may not be needed and it may unlock after completing the next game. I advise removing the save files if you were at 100 games played before the update. If you haven't done it, please follow Yurgi Windu's excellent guide.

Set up a Knockout Cup with two teams, one-legged. Add any AI manager.

Create a new manager and a new team.

Skip all picks and then play the game. I drew 0-0 and won on penalties.

Popped after the game. I had completed 100 games before but it unlocked after 1 game for the person who won this first (not me). That person was new to the game and had no other save files.

This will need more feedback and testing, please comment below what does and does not work.
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This achievement is currently unobtainable.

I have reported it to Sports Interactive. This is how I worked with the devs to get the fixes last year. Please post in that forum to help get a fix ASAP. You can see in the evidence posted at the link below, that I have played 100 matches in this mode.

https://community.sigames.com/topic/518844-game-pass-bugs/

This achievement has now been patched and is no longer unobtainable. It can be popped in a couple of hours with an AFK method as follows. Just make sure your game is up to date and has the latest version installed. Mine is version 20.4.1

On the main menu select Fantasy Draft --> Create Draft. Change the format to League, Draft Time limit to 1 minute and move the slider for matches against each team to 5. Next hit create competition.

Next create your manager and team (any settings will do) and on the screen where it is asking to add manager click Add AI club and keep adding clubs until there are 20 teams including your own team. Next begin draft.

Here you can do whatever you like, we don't need to win any games we just need to play them, so for quickness you can just select "complete draft" and it will select players for you. Either that or you can pick your own. Once all teams have selected their players (this can take a few minutes) you are now ready for the idling.

On the main page after the game saves simply go to the little home button icon then select "Actions" and then "Go on Holiday". Tick go on holiday indefinitely and then hit "go on holiday". Sit back and the game will run through 5 matches against 19 other teams which = 95 matches.

This can be done across multiple saves and teams so it doesn't need to be done in one sitting. For that reason I would recommend going for knockout king first
since the matches played in that will count towards this achievement.

If at any point the league ends and you have not got the 100 matches yet just run it again until it pops. One thing of note is that after about 40 matches or so a full year is completed so the league starts again automatically but don't worry it just keeps idling away.