How to cite an online lecture in MLA?

I'd have said, use @misc and add fields so that things look OK, but there is a small issue with @misc entries in mla-new at the moment (https://github.com/jmclawson/biblatex-mla/issues/23), so I suggest you take @online.

The following seems to give acceptable results

documentclass[american]{article}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage{babel}
usepackage{csquotes}

usepackage[style=mla-new, backend=biber]{biblatex}


begin{filecontents}{jobname.bib}
@online{elk,
  author    = {Anne Elk},
  title     = {Lecture on a Theory on Brontosauruses},
  date      = {1972-11-16},
  publisher = {University of Python},
  location  = {Monty},
  note      = {Online lecture},
}
end{filecontents}
addbibresource{jobname.bib}
addbibresource{biblatex-examples.bib}


begin{document}
Lorem autocite{sigfridsson}
ipsum autocite{elk}
printbibliography
end{document}

If the recording of the lecture is available via some route, you may want to include the relevant information in the note field as well.