Hi @Yuki.O,
As you have probably realised, you can’t use an input box to combine values.
You can use the logged in user for this…
- Make a text field in user table (I have just used ‘Username’)
- Click Action updates the Logged in username = #tag & ‘LIU Username’
- Input default value = LIU Username
When you submit the form or first come to the page you may want to reset the value of that input, so you must reset the ‘LIU Username’ - or whatever text field you used.
Nocode monkey also had a ‘tag’ style input too. Search ‘Mentions’ on component marketplace.
Edit: I would run the hashtags from a list, same concept applies. You could also update the ‘current message’ instead of Logged in user. In this case you need to create the message on the button that navigates to the message input screen.
Good luck 
= 