Submitting co-signed LoR through interfolio?

[HerbalMedicine]

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How does this work? My TA agreed to write a letter for me and have it co-signed by the professor. However, how can the professor co-sign it if it's to be submitted electronically?

 

[grandslamsam]

I think it would work best to have the TA write the letter and then have the professor also put their name at the bottom of the letter. After that, have the professor submit it through their account.