If you have not connected any stores to BigSeller:

After signing up with BigSeller, you will be directed to Dashboard Wizard Setup
 

Step 1:

Select Lazada and Click on Connect Store,  input the store nickname and click on Connect Store

 

Step 2:

After that, you will be redirected to lazada and you'll be prompted to log in to your account.  

Step 3:

Complete login to your marketplace account  Click on Yes and accept the authorization

If you would like to connect new stores to BigSeller:
 

Step 1:

Click on Integration > Click on Add New Store

 

Step 2:

Input the store nickname and click on Connect Store




Step 3:

Just follow theStep 2 and Step 3 of the first method above