How do I find the grantee for a specific AWS user?

In IAM Management Console, I created a user.

I want to give to this user permissions to upload files to S3 bucket.

In bucket properties, Permissions tab asks to provide a grantee and check/uncheck the permissions such as list or upload/delete. I unsuccessfully tried to enter the following in the grantee field:

• User ARN.
• User name.
• Access key.
• Numeric part of user's ARN.

In every case, the same error is shown:

How do I find the grantee value corresponding to a specific user?