MySQL: How to set the default string using another field by string-interpolation?
This is possible in MySQL 8.0 because they support expressions in DEFAULT clauses now. But it won't work with interpolation like you show. It works like any other MySQL string expression:
CREATE TABLE Account (
 AccountID varchar(25),
 About varchar(300) not null default (CONCAT('My id is ', AccountID)),
 Pagelink varchar(255) not null unique
);

Example:
mysql&gt; INSERT INTO Account SET AccountID = '8675309', Pagelink = '&lt;link&gt;';
Query OK, 1 row affected (0.00 sec)

mysql&gt; select * from Account;
+-----------+------------------+----------+
| AccountID | About            | Pagelink |
+-----------+------------------+----------+
| 8675309   | My id is 8675309 | &lt;link&gt;   |
+-----------+------------------+----------+

In earlier versions of MySQL, defaults can only be a literal value, or NULL, or CURRENT_TIMESTAMP, but not an expression.