As a workaround, I used the command line to create the store:

xe sr-create host-uuid=<your_host_uuid> content-type=iso type=iso shared=true name-label="SMB ISO SR" device-config:location="\\\\10.10.10.139\\share" device-config:type=cifs device-config:username=<username>

Note the escaped backslash. And in my case I didn't need an SMB password but the command line insisted on me providing a username.

And as a side note, getting the command line wrong still seems to end up creating broken SR entries that need to be cleaned.

I'm seeing this same issue, there is no "Create" button appearing in the Summary pane.

What format is the SMB Server address expected to be in? Windows style as with the OP?

If I look at the browser dev tools, it seems to be spitting out formatting errors relating to the server string:

index.js:525 [React Intl] Error formatting the default message for: "newSrSmbAddressPlaceHolder"
SyntaxError: Expected "\\#", "\\\\", "\\u", "\\{", "\\}", "{", [^{}\\\0-\x1F\x7f \t\n\r], end of input or whitespace but "\\" found.
J @ index.js:525