First you have to hide the database list from the openerp interface.

For that you have to edit the below files.

base.xml (path : addons/web/static/src/xml/base.xml)

<t t-name="Login.dblist">

<select name="db">

<t t-foreach="db_list" t-as="db">

<t t-if="selected_db === db">

<option t-att-value="db" selected="true">

<t t-esc="db"/>

</option>

</t>

<t t-if="selected_db !== db">

<option t-att-value="db">

<t t-esc="db"/>

</option>

</t>

</t>

</select>

</t>

Now replace that selection "db" with a text box with the same name of the selection field as

<t t-name="Login.dblist">

<input name="db" type="text" value="" autofocus="autofocus"/>

</t>

EDIT :

Then in chrome.js (addons/web/static/src/js/chrome.js) comment the on_db_loaded method (line 676-690) , which is

on_db_loaded: function (result) {

     var self = this;

     this.db_list = result;

     if (!this.selected_db) {

        this.selected_db = result[0];

     }

     this.$("[name=db]").replaceWith(QWeb.render('Login.dblist', { db_list: this.db_list, selected_db: this.selected_db}));

     if(this.db_list.length === 0)

     { this.do_action("database_manager");

     }

     else if(this.db_list.length === 1)

     { this.$('div.oe_login_dbpane').hide();

    }

    else { this.$('div.oe_login_dbpane').show(); } },

comment the above full code.

For this refer the link 

Then provide link to each client. When they click their link, it automatically opens their database without selecting the db's.