Hi yvka123,
As Viktor suggested, you can set up a trigger for when a new record (Post) is created, user's id stores in the text of the post.
How to filter the text of the post to get the ID of the mentioned user?
--Using split() function twice in the flow:
Overview:

Steps:
1.Trigger:

2.Get a row by id:

Then you can test run the flow to view format of the text:

Special format: @[8,userid,username,...], So you can use these special characters to split the text.
3.Using split() firstly to split text got from last step:
split(outputs('Get_a_row_by_ID')?['body/largetext'], '@[8,')[1]
4.use split() again to split output got from last step:
split(outputs('Compose'),',')[0]

Reference(usage of split() function in power automate):
https://www.enjoysharepoint.com/power-automate-split-string-into-an-array/
Then you can use the id of the user in your following steps to send in-app notification.