PostgreSQL - How to make Primary Key have X number of digits?

I am fairly new to SQL but have been working hard to learn. I am currently stuck on an issue with setting a Primary Key to have 8 digits no matter what.

I tried doing INT(8) but that didn't work. Also AUTO_INCREMENT doesn't work in PostgreSQL but I saw there were a couple of datatypes that auto increment but I still have the issue of the keys not being long enough.

Basically I want to have numbers represent User IDs, starting at 10000000 and moving up. 00000001 and up would work too, it doesn't matter to me.

I saw an answer that was close to this, but it didn't apply to PostgreSQL unfortunately.

Hopefully my question makes sense, if not I'll try to clarify.

Thank you!

My code (which I am using from a website to try and make my own forum for a practice project) is:

CREATE Table users (
user_id     INT(8) NOT NULL AUTO_INCREMENT,
user_name   VARCHAR(30) NOT NULL,
user_pass   VARCHAR(255) NOT NULL,
user_email  VARCHAR(255) NOT NULL,
user_date   DATETIME NOT NULL,
user_level  INT(8) NOT NULL,
UNIQUE INDEX user_name_unique (user_name),
PRIMARY KEY (user_id)

) TYPE=INNODB;

It doesn't work in PostgreSQL (9.4 Windows x64 version). What do I do?