I found the solution:
1 - On "Power Automate", create a new "instant cloud flow", find Teams action: "From the compose box".
2 - Click "Edit adaptive card". You now need to get a bit familiar with JSON and adaptive cards. This builder is useful and better than the one in Power Automate:
https://adaptivecards.io/designer/
3 - Be aware that each "input" block needs to have an "Id". When you're done, copy and paste the JSON code from adaptivecards.io if you used it, into the default designer.
4 - Click on "Next step" and Search for Teams action: "Post a message in a chat or channel".
5 - Now in this section you need to decide if the post will be as "User", or "Flow bot". I chose "User". "Post in" can be "Group chat" or "Channel". Then choose your "Team" and "Channel" (if you chose channel). On "Message" you should switch to HTML mode and use simple HTML for formatting the text that gets posted on the channel. For example, you can type the name for each item/text box on the form and use <strong>
for bold and <br>
for a line break and so on. Now in HTML mode, you can use the "Id" mentioned in point 3, which should be used here in order to get the value/text from the form the user submitted. You can choose the "Id"s on a panel that pops up on the right.
6 - Now this next points I don't remember exactly. I'm not going to go back and reconfigure this on my Teams just to know what to write here, so I'll just explain as I remember it. On the Windows Teams app (or using Teams in the browser), you will need to install or add the "Power Automate" app. Then you also need to install/add the "Workflows" app.
7 - Go to a compose text box in any chat, group chat, or channel that is not private, click the 3 dots on the right (last icon on the compose box), and search for "Workflows" after you find it, right-click on it and "pin" it to the compose box. Now every time you want to open the form just click on "Workflows" and then choose the form. If you end up creating more forms using Teams action "From the compose box" (mentioned in point 1), all these forms will show up here as a list, after you click on "Workflows" because that's what they are: Workflows.
8 - I think there is an extra step that I don't remember well where you might need to connect your Teams to Power Automate through a connector or something, but now I'm not sure. Anyway, these last points from 6 to 8 were easy to figure out. The hardest part for me was from points 1 to 5 where I did need to dig for a few days to get everything working. If you have any questions just comment below. If you remember exactly what the steps are from 6 to 8, or if you find anything that needs correcting, please let me know in the comments too. Thanks! :)