The 82-year-old veteran actress played Queen Vic landlady Peggy Mitchell for over 20 years and Dyer joined the BBC soap in 2013 as new pub landlord Mick Carter.
Dyer revealed on new series Secrets From The Square: "When I got the part – and I'd never met her – she sent me a postcard giving me her blessing that I could have [the Queen Vic]. And I was like, 'Oh, wow'.”
He then teased co-star Kellie Bright, who plays his onscreen wife Linda Carter: “And Kellie was like, 'I didn't get one of them!' and got the right hump about it."
Windsor last appeared on EastEnders in 2016. She was diagnosed with dementia in 2014 and her husband Scott Mitchell, 57, recently opened up about his heartbreak as her health has continued to deteriorate.
Dyer and Bright appeared together – sitting two metres apart in accordance with social distancing guidelines – talking to host Stacey Dooley about their time on the soap as part of the new series, which has been recorded during lockdown.
Bright said of social distancing: "You just want to hug people. I just want to hug [Danny]! I wanted to give him a squeeze and say, 'Oh my gosh! It's so good to see you!'
"I think it's the first time in seven years apart from maternity leave that I haven't seen Danny in all that time."
Dyer quipped: "Yeah. It's been hard to carry on the affair..."
He added: “What?!... Too much?”
Secrets From The Square has launched on BBC One a week after EastEnders went off air for the first time since it began in 1985.
The last episode of the BBC soap aired on Tuesday, 16 June and production has not resumed.
Producers are hoping to start filming in Albert Square again later this month and when it does return, episodes will be 10 minutes shorter than before.
The show had already been airing at a reduced number of episodes a week in order the stretch out what remained for as long as possible in lockdown.
Other EastEnders stars who have already been revealed to be taking part include Letitia Dean, Tameka Empson and Adam Woodyatt.
The series continues on Mondays at 8pm on BBC One.