Help with steem-python's TransactionBuilder?

I am currently working on a soft GUI wallet (keys are not stored) using python. I am working with steem-python's TransactionBuilder and I am having some trouble:

Here is the code that I am using to send a transaction:

First, I set up the transfer and the TransactionBuilder and then use operations.Transfer() to make my transfer readable for the TransactionBuilder. Next, I use appendMissingSignatures() to add the WIF active key that the user has provided to the TransactionBuilder, and then attempt to sign and broadcast the transaction (no_broadcast = True, as I am just testing).

Obviously, the way that I am adding the WIF key to the TransactionBuilder is incorrect, as the following MissingKeyError is raised:

What am I doing wrong here? I appreciate your help.