Uploading a video to a Squarespace site

https://duckduckgo.com/?q=html+video+tag&t=ffhp&ia=web

Premiere Pro project files are the working instructions for a project, not a video itself. So you always export your project to a video format (wrapper) and codec according to the needs for that use. You can (and typically will) export projects in different settings for different uses.

I think Squarespace has a lot of help material available ... check with them. But if they want something usable on YouTube, as in, exported in a format & codec that would work on YouTube ... I'd suggest using a DNxHD/R codec. Those will work to upload to YouTube, and don't seem to have the gamma/dynamic range issues that some PrPro users get uploading H.264 format videos to YouTube.

You can actually use DNxHD/R in a couple different formats ... in the MXF-Op1a format (gets a .mxf extension to the file), or a QuickTime ... (gets a .mov extension). If you try the QuickTime format one, select QuickTime in the Format section, then skip down past the preset options straight to the Video tab below in the Export Settings dialog. In the Codec drop-down list, near the top is the DNxHD/R option ... and then after setting that, you've got the options in "Resolution" to get the frame-size for your needs.

Neil