Enforce a knowledge article template at the time of creation
In the Knowledge Base, you can choose a default field and choose what you want to put in that default field. For instance, you can choose "Text" and put in what text you want to be in the article when it is created. A couple of drawbacks. This does not support HTML and the user still has to click OK to apply the default text you chose.
On the "Text" field on the Knowledge Article form, you can right-click on this field and choose "Configure Dictionary". From there, go to the default tab and you will see a text box. You can put whatever text you want in here and HTML is supported. This will fill in the field when an article is created.
Out of Box, we do offer Knowledge Templates in the Template bar that you can create but users have to click on the template. But you can create a script that calls a Knowledge Article Template and loads it into the article. This will require more customization but there is documentation about calling templates in Scripts https://docs.servicenow.com/bundle/jakarta-platform-administration/page/administer/form-administration/reference/r_ScriptedTemplates.html. This is also mentioned on the Community. https://community.servicenow.com/community?id=community_question&sys_id=3e7047e1db98dbc01dcaf3231f9619a7&view_source=searchResult. If you choose this option, we would suggest posting any issues in the community as this is the best place that can help with customization.