One or more sources can be allowed for the base-uri policy:

Content-Security-Policy: base-uri <source>;
Content-Security-Policy: base-uri <source> <source>;

This directive uses most of the same source values for arguments as other CSP directives: CSP Source Values.

Note however that some of the values don't make sense for base-uri, such as the keywords 'unsafe-inline' and 'strict-dynamic'.

<IfModule mod_headers.c>
Header set Content-Security-Policy "base-uri 'self'";
</IfModule>

add_header Content-Security-Policy "base-uri 'self';"

Since your domain isn't example.com, a <base> element with its href set to https://example.com will result in a CSP violation.

<meta http-equiv="Content-Security-Policy" content="base-uri 'self'" />
<base href="https://example.com/" />

<!--
// Error: Refused to set the document's base URI to 'https://example.com/'
// because it violates the following Content Security Policy
// directive: "base-uri 'self'"
-->

• Content-Security-Policy
• <base>
• Node.baseURI