I am so sorry, and please forgive my ignorance. I understand, html, css, php and I can program for wordpress - but I dont understand what you mean by "add http://YourTumblrName.tumblr.com/post/ to this ID and you will get the link to the post."
How do i do that? I created a custom field - but I dont know how to use the "pgID";d:73517101489 as you instructed. I do see that the number 73517101489 is the actual link to the post that I am testing on - but I dont know how you got that number. Its quite unique, and I didnt give it to you.
And if i need to make a new custom field for every single new post - thats not going to work.
Thanks for all your help.