$ curl -v https://self-signed.badssl.com/
    *   Trying 104.154.89.105:443...
    * Connected to self-signed.badssl.com (104.154.89.105) port 443 (#0)
    * ALPN, offering h2
    * ALPN, offering http/1.1
    *  CAfile: /etc/ssl/certs/ca-certificates.crt
    *  CApath: /etc/ssl/certs
    * TLSv1.0 (OUT), TLS header, Certificate Status (22):
    * TLSv1.3 (OUT), TLS handshake, Client hello (1):
    * TLSv1.2 (IN), TLS header, Certificate Status (22):
    * TLSv1.3 (IN), TLS handshake, Server hello (2):
    * TLSv1.2 (IN), TLS header, Certificate Status (22):
    * TLSv1.2 (IN), TLS handshake, Certificate (11):
    * TLSv1.2 (OUT), TLS header, Unknown (21):
    * TLSv1.2 (OUT), TLS alert, unknown CA (560):
    * SSL certificate problem: self-signed certificate
    * Closing connection 0
    curl: (60) SSL certificate problem: self-signed certificate
    More details here: 
    https://curl.se/docs/sslcerts.html

    $ curl --version
    curl 7.81.0 (x86_64-pc-linux-gnu) libcurl/7.81.0 OpenSSL/3.0.2 zlib/1.2.11 brotli/1.0.9 zstd/1.4.8 libidn2/2.3.2 libpsl/0.21.0 (+libidn2/2.3.2) libssh/0.9.6/openssl/zlib nghttp2/1.43.0 librtmp/2.3 OpenLDAP/2.5.13
    Release-Date: 2022-01-05
    Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp 
    Features: alt-svc AsynchDNS brotli GSS-API HSTS HTTP2 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM NTLM_WB PSL SPNEGO SSL TLS-SRP UnixSockets zstd

        curl -kv https://self-signed.badssl.com/
        *   Trying 104.154.89.105:443...
        * TCP_NODELAY set
        * Connected to self-signed.badssl.com (104.154.89.105) port 443 (#0)
        * ALPN, offering http/1.1
        * Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
        * successfully set certificate verify locations:
        *   CAfile: /opt/local/share/curl/curl-ca-bundle.crt
        CApath: none
        * TLSv1.2 (OUT), TLS header, Certificate Status (22):
        * TLSv1.2 (OUT), TLS handshake, Client hello (1):
        * TLSv1.2 (IN), TLS handshake, Server hello (2):
        * TLSv1.2 (IN), TLS handshake, Certificate (11):
        * TLSv1.2 (IN), TLS handshake, Server key exchange (12):
        * TLSv1.2 (IN), TLS handshake, Server finished (14):
        * TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
        * TLSv1.2 (OUT), TLS change cipher, Change cipher spec (1):
        * TLSv1.2 (OUT), TLS handshake, Finished (20):
        * TLSv1.2 (IN), TLS change cipher, Change cipher spec (1):
        * TLSv1.2 (IN), TLS handshake, Finished (20):
        * SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
        * ALPN, server accepted to use http/1.1
        * Server certificate:
        *  subject: C=US; ST=California; L=San Francisco; O=BadSSL; CN=*.badssl.com
        *  start date: Aug 12 15:59:10 2022 GMT
        *  expire date: Aug 11 15:59:10 2024 GMT
        *  issuer: C=US; ST=California; L=San Francisco; O=BadSSL; CN=*.badssl.com
        *  SSL certificate verify result: self signed certificate (18), continuing anyway.
        > GET / HTTP/1.1
        > Host: self-signed.badssl.com
        > User-Agent: curl/7.65.1
        > Accept: */*
        > 
        * Mark bundle as not supporting multiuse
        < HTTP/1.1 200 OK
        < Server: nginx/1.10.3 (Ubuntu)
        < Date: Fri, 21 Oct 2022 18:41:58 GMT
        < Content-Type: text/html
        < Content-Length: 502
        < Last-Modified: Fri, 12 Aug 2022 15:59:21 GMT
        < Connection: keep-alive
        < ETag: "62f678d9-1f6"
        < Cache-Control: no-store
        < Accept-Ranges: bytes
        <

If you decide you trust that certificate (which can be a legitimate thing to do - the cert signature could be communicated to you via out-of-band trusted mechanisms) then https://curl.se/docs/sslcerts.html explains how to trust it.