Both users and groups are considered "namespaces" as far as the Gitlab API is concerned, so you can use either a group ID or a username in the namespace_id field. You can see this in use by getting a single namespace with either a Group ID (that you can see in the /groups API call, or from a Group's "profile" page) or a username:

# this will show the namespace details of the Group with ID 54
curl --header "PRIVATE-TOKEN: ${TOKEN}" "https://gitlab.com/api/v4/namespaces/54

# this will show the namespace details of the User with username my-username
curl --header "PRIVATE-TOKEN: ${TOKEN}" "https://gitlab.com/api/v4/namespace/my-username

Since you just wanted to create a repo, there is no need of third party apps. You can directly send a post request to gitlab API which will create repo.

Go to account tab in your profile, you will find a private token. Copy that.

Now open terminal and run this command with private token (say foo) and your repo name (say bar).

curl -H "Content-Type:application/json" https://gitlab.com/api/v4/projects?private_token=foo -d "{ \"name\": \"bar\" }"

For convenience, you can create a shell script, if you don't want to run this command every time.

#!/bin/sh

curl -H "Content-Type:application/json" https://gitlab.com/api/v4/projects?private_token=foo -d "{ \"name\": \"$1\" }"

Save this to a file gcr.sh and make it executable using chmod +x gcr.sh.

Now to create a repo name bar, run

$ ./gcr.sh bar