How do I insert data into a table where one of the values should be a foreign key from another table?

I wanted to update the post table (main table) with those data:

User: "helper"
Title: "Title"
Description: "Description"

However, the post table userid column only accepts an integer (foreign keys from the user table (userid)), not username string itself.

How can I do this with INSERT, SELECT/WHERE query?

There are the tables.

post

+--------+--------+---------+-------------+  
| postid | userid | title   | description |  
+--------+--------+---------+-------------+  
| 1      | 1      | example | example     |  
+--------+--------+---------+-------------+  
| 2      | 2      | example | example     |  
+--------+--------+---------+-------------+  
| 3      | 3      | example | example     |  
+--------+--------+---------+-------------+  

user

+--------+----------+--------+  
| userid | username | roleid |  
+--------+----------+--------+  
| 1      | admin    | 1      |  
+--------+----------+--------+  
| 2      | helper   | 1      |  
+--------+----------+--------+  
| 3      | test     | 2      |  
+--------+----------+--------+  

Here is what it should look like after the insertion:

post

+--------+--------+---------+-------------+  
| postid | userid | title   | description |  
+--------+--------+---------+-------------+  
| 1      | 1      | example | example     |  
+--------+--------+---------+-------------+  
| 2      | 2      | example | example     |  
+--------+--------+---------+-------------+  
| 3      | 3      | example | example     |  
+--------+--------+---------+-------------+
| 4      | 2      | Title   | Description |  
+--------+--------+---------+-------------+    

Answer

Assuming postid auto-increments:

insert into post (userid, title, description)
select userid, "Title", "Description"
from user
where username="helper"

Note that this will not insert anything if the given username is not found.