Inject the code to yammer page, it will automatically save data in localStorage and load it when the page is loaded. For preventing data lost in unexpected page closing.

yammer-draft.js

var storage = {
    key: 'yammer_draft',
    save: function(data) {
        localStorage.setItem(this.key, JSON.stringify(data))
    },
    get: function() {
        return JSON.parse(localStorage.getItem(this.key))
    }
}

var getInputElement = function() {
    return document.getElementsByClassName('yj-tapf-textarea')[0]
}

var getDraftData = function() {
    var inputElement = getInputElement()
    return (!!inputElement) ? inputElement.value : ''
}

var activateTextArea = function() {
    var elementWrapper = document.getElementsByClassName('yj-global-thread-starter-publisher')[0]
    var elementButton = elementWrapper.getElementsByClassName('publisher-placeholder')[0]
    elementButton.click()
    
    return getInputElement()
}

// Bind event that saves data as draft when closing page
window.addEventListener('beforeunload', function(){
    var data = getDraftData()
    if (data.length > 0) {
	    storage.save(getDraftData())
    }
})

// Inject draft when document is loaded
window.addEventListener('load', function(){
    console.log('>> yammer_draft: waiting...')
    var draftData = storage.get()
    setTimeout(function(){
        try {
            var inputElement = activateTextArea()
            inputElement.value = draftData
            inputElement.click()
            console.log('>> yammer_draft: loaded')
        } catch(e) {
            console.warn('>> yammer_draft: An error occured, cannot interact with DOM')
            console.log('>> DRAFT_DATA:', draftData)
        }
    }, 6000)
})