openGreetings.cmp :
<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global">
    <aura:attribute name="pageReference" type="Object"/>
    <lightning:workspaceAPI aura:id="workspace"/>
    <lightning:button label="Open Greeting in Subtab" onclick="{!c.openSubtab}"/>
    <lightning:input label="Name" name="myname"/>
</aura:component>

openGreetingsController.js :
({
    openSubtab: function(component, event, helper) {
        var workspaceAPI = component.find("workspace");
        workspaceAPI.getEnclosingTabId().then(function(enclosingTabId) {
            workspaceAPI.openSubtab({
                parentTabId: enclosingTabId,
                pageReference: {
                    "type": "standard__component",
                    "attributes": {
                        "componentName": "c__greetings"
                    },
                    "state": {
                        "uid": "1",
                        "c__name": component.get("v.myName")
                    }
                }
            }).then(function(subtabId) {
                console.log("The new subtab ID is:" + subtabId);
            }).catch(function(error) {
                console.log("error");
            });
        });
    }
})


 greetings.cmp : 

<aura:component implements="lightning:isUrlAddressable">
    <aura:attribute name="name" type="String" description="The person that will be greeted" />
    <aura:handler name="init" value="{!this}" action="{!c.init}" />
    <aura:handler name="change" value="{!v.pageReference}" action="{!c.handlePageChange}" />
    <h1>Greeting Page</h1>
    <div>Hello, {!v.name}</div>
</aura:component>

greetingsController.js : 

({
    init: function(cmp, evt, hlp) {
        var myPageRef = cmp.get("v.pageReference");
        var name = myPageRef && myPageRef.state ? myPageRef.state.c__name : "World";
        cmp.set("v.name", name);
    },
    handlePageChange: function(cmp, evt, hlp) {
        var myPageRef = cmp.get("v.pageReference");
        var name = myPageRef && myPageRef.state ? myPageRef.state.c__name : "World";
        cmp.set("v.name", name);
    }
})

Getting error is  = Error: Invalid or missing parentTabId `false`