Is it possible ?
Yes, the user would insert the username and password into your form. And you would perform behind the curtains a call to a Keycloak Client that you would have to configure with the "Resource Owner Password Credential Grant" flow (i.e., Direct access Grants Enabled in Keycloak). That call would be requesting a token on the user's behalf.
Now the problem with this approach is that Resource Owner Password Credential Grant should typically be avoided (source):
The resource owner password credentials grant type is suitable in
cases where the resource owner has a trust relationship with the
client, such as the device operating system or a highly privileged
application. The authorization server should take special care when
enabling this grant type and only allow it when other flows are not
viable.
You can read more on why it is not recommended to use that flow in sources such as this.
So you would be compromising a bite of security there. Moreover, you might want to provide other Keycloak functionality (e.g., OTP and social media login) that is seamlessly integrated with its Theme template. Then you might have to adapt your application based on new Keycloak releases and so on. Unless you really have a very good reason to I would not recommend it. You can opt to simply customize the default theme. It is just a form anyway, after that the user can be immediately redirected to your app.