Follow below mentioned steps to get your Flickr username:
Step 1: Go to Flickr (https://www.flickr.com/)
Step 2: Click on Sign Up to create a profile on Flickr. If you already own a Flick account, click login, skip step 3 and continue following tutorial from step 4.
Step 3:
(i) You will be redirected to Yahoo Sign Up Page, enter all of the mandatory information, and click on Continue
(ii) Check your email address and click on Send me an Account Key button
(iii) Enter the account key and click on Continue button to verify your email address
(iv) Click on Continue button
(v) Now click on Next button to setup your Flickr profile
Step 4:You will be redirected to Yahoo login page, enter your email address and click on Next
Step 5: This is your Flickr Dashboard, click on your profile icon
Step 6: This is your Flickr username, copy and paste it in the respective field on app builder