Hi, @amora1128 ????
If you’re using a block-based theme, you could add a custom graphic using the Image block, or an icon-dedicated block such as that offered by The Icon Block plugin (which supports native WP icons and custom SVGs). Then link the image/icon to your contact form (e.g. to https://example.com/contact-us/
).
There are also options available for classic themes, but those may require additional steps and custom code. This depends on your theme setup.
If your online chat functionality is supplied through a plugin, then it may already include a block or shortcode to add a link to the site. These can often be customized to match your needs.