When I try to add a subscription using: https://netstream.pushpad.xyz/p/3630?ui=false/edit?uid=admin&uid_signature=xxxxxxxxxxxxxx
the user gets registered but the User ID column in the Subscriptions Tab of my Dashboard shows null. Please advice as to why this is not getting updated correctly.
That link is incorrect. Try with:
https://netstream.pushpad.xyz/p/3630?ui=false&uid=admin&uid_signature=xxxxx
If that doesn't work again, you must make sure that the signature is correct.