getting user id of the user whenever the email address of the user is entered

Anonymous0909 , You can use API V1 to fetch user record based on there email id.

Below is the API at the end you need to add actual {email} of the user, this will return the information of that user.

https://community.lithium.com/restapi/vc/users/email/{email}

<response status="success">
  <user type="user" href="/users/id/43">
    <registered type="boolean">false</registered>
    <id type="int">43</id>
    <deleted type="boolean">false</deleted>
    <email type="string">john.doe@example.com</email>
    <anonymous type="boolean">false</anonymous>
    <login type="string">john_doe</login>
    <sso_id type="string">johndoe47</sso_id>
    <registration_time type="date_time">2006-07-18T22:13:32+00:00</registration_time>
    <last_visit_time type="date_time">2006-08-12T11:14:04+00:00</last_visit_time>
    <average_message_rating type="float">4.6</average_message_rating>
    <average_rating type="float">3.9</average_rating>
    <profiles>
      <profile name="name_last" type="string">Doe</profile>
      <profile name="name_first" type="string">John</profile>
    </profiles>
    <banned type="boolean">false</banned>
  </user>
</response>

From the response you can get ID of the user.

<id type="int">43</id>